Add prettier opinionated formatter

.github/fix-prettier-14007-no-unstable-semicolon.patch

@@ -0,0 +1,16 @@
+diff --git a/src/language-css/printer-postcss.js b/src/language-css/printer-postcss.js
+index 4ba444c8d..eaa5e38ff 100644
+--- a/src/language-css/printer-postcss.js
++++ b/src/language-css/printer-postcss.js
+@@ -101,7 +101,10 @@ function genericPrint(path, options, print) {
+       return [node.raw, hardline];
+     case "css-root": {
+       const nodes = printNodeSequence(path, options, print);
+-      const after = node.raws.after.trim();
++      let after = node.raws.after.trim();
++      if (after.startsWith(";")) {
++        after = after.slice(1).trim();
++      }
+
+       return [
+         nodes,

.github/fix-prettier-14008-no-space-after-unary-minus.patch

@@ -0,0 +1,23 @@
+diff --git a/src/language-css/printer-postcss.js b/src/language-css/printer-postcss.js
+index 4ba444c8d..a15f7981f 100644
+--- a/src/language-css/printer-postcss.js
++++ b/src/language-css/printer-postcss.js
+@@ -744,6 +744,18 @@ function genericPrint(path, options, print) {
+           continue;
+         }
+
++        // No space before unary minus followed by an opening parenthesis `-(`
++        if (
++          (options.parser === "scss" || options.parser === "less") &&
++          isMathOperator &&
++          iNode.value === "-" &&
++          isParenGroupNode(iNextNode) &&
++          locEnd(iNode) === locStart(iNextNode.open) &&
++          iNextNode.open.value === "("
++        ) {
++          continue;
++        }
++
+         // Add `hardline` after inline comment (i.e. `// comment\n foo: bar;`)
+         if (isInlineValueCommentNode(iNode)) {
+           if (parentNode.type === "value-paren_group") {

.github/fix-prettier-14034-no-property-name-lower-if-inside-variable-declaration.patch

@@ -0,0 +1,18 @@
+diff --git a/src/language-css/printer-postcss.js b/src/language-css/printer-postcss.js
+index 4ba444c8d..431cdc7a6 100644
+--- a/src/language-css/printer-postcss.js
++++ b/src/language-css/printer-postcss.js
+@@ -155,7 +155,12 @@ function genericPrint(path, options, print) {
+
+       return [
+         node.raws.before.replace(/[\s;]/g, ""),
+-        insideICSSRuleNode(path) ? node.prop : maybeToLowerCase(node.prop),
++        (parentNode.type === "css-atrule" &&
++          typeof parentNode.raws.params === "string" &&
++          parentNode.raws.params.startsWith(":")) ||
++        insideICSSRuleNode(path)
++          ? node.prop
++          : maybeToLowerCase(node.prop),
+         trimmedBetween.startsWith("//") ? " " : "",
+         trimmedBetween,
+         node.extend ? "" : " ",

.github/fix-prettier-value-with-comma.patch

@@ -0,0 +1,18 @@
+diff --git a/src/language-css/printer-postcss.js b/src/language-css/printer-postcss.js
+index 4ba444c8d..4f7738bfa 100644
+--- a/src/language-css/printer-postcss.js
++++ b/src/language-css/printer-postcss.js
+@@ -886,11 +886,11 @@ function genericPrint(path, options, print) {
+
+       if (!node.open) {
+         const printed = path.map(print, "groups");
+-        const res = [];
++        const res = [hardline];
+
+         for (let i = 0; i < printed.length; i++) {
+           if (i !== 0) {
+-            res.push([",", line]);
++            res.push([",", hardline]);
+           }
+           res.push(printed[i]);
+         }

.github/workflows/ci.yml

@@ -11,9 +11,19 @@ jobs:
     - uses: actions/checkout@v3
     - uses: actions/setup-node@v3
     - name: Install dependencies
-      run: yarn && yarn add -D eslint-config-airbnb-base eslint-plugin-unicorn eslint-plugin-jest@^26.9.0 @typescript-eslint/eslint-plugin @typescript-eslint/parser typescript
+      run: >
+        yarn
+        && yarn add -D eslint-config-airbnb-base eslint-plugin-unicorn eslint-plugin-jest@^26.9.0 @typescript-eslint/eslint-plugin @typescript-eslint/parser typescript github:prettier/prettier#^2.8.1
+        && (cd node_modules/prettier && git apply -v ../../.github/fix-prettier-14007-no-unstable-semicolon.patch)
+        && (cd node_modules/prettier && git apply -v ../../.github/fix-prettier-14008-no-space-after-unary-minus.patch)
+        && (cd node_modules/prettier && git apply -v ../../.github/fix-prettier-14034-no-property-name-lower-if-inside-variable-declaration.patch)
+        && (cd node_modules/prettier && git apply -v ../../.github/fix-prettier-value-with-comma.patch)
     - name: Lint JS and LESS files
-      run: npm run lint
+      run: >
+        npm run lint
+        && npx prettier --loglevel warn '!dist' '!test/coverage' '!src/semantic.less' '**/*.{css,less,overrides,variables}' --write
+        && git restore package.json yarn.lock
+        && git add . -N && git diff --exit-code
   test:
     name: Test build process on node ${{ matrix.node-version }}
     runs-on: ubuntu-latest

.prettierrc.js

@@ -0,0 +1,12 @@
+module.exports = {
+    tabWidth: 4,
+    printWidth: 120,
+    overrides: [
+        {
+            files: ['*.less', '*.overrides', '*.variables'],
+            options: {
+                parser: 'less',
+            },
+        },
+    ],
+};

.stylelintrc.js

@@ -28,6 +28,13 @@ module.exports = {
         'string-quotes': 'double',
         'value-keyword-case': null,
 
+        // fix compatibility with Prettier
+        'declaration-bang-space-before': null,
+        'declaration-block-semicolon-space-before': null,
+        'declaration-colon-newline-after': null,
+        'declaration-empty-line-before': null, // TODO
+        'selector-combinator-space-before': null,
+
         // TODO rules to be removed/fixed in v2.10.0 as fixes are not compatible with IE11
         'alpha-value-notation': 'number', // https://caniuse.com/mdn-css_properties_opacity_percentages