build(package): switch to tsup

Changes a few things:
- replaces custom build script with faster & better `tsup`
- adds `exports` to `package.json`
- switches to `pnpm` from `npm`

BREAKING CHANGE: users of this library will no longer be able to `require`/`import` anything from  a subpath, i.e. `decaffeinate-parser/dist/*`. This was never an intentional part of the API, but it was available and decaffeinate used it.

.circleci/config.yml

@@ -2,20 +2,22 @@ aliases:
   install_dependencies_step: &install_dependencies_step
     run:
       name: install-dependencies
-      command: npm ci
+      command: |
+        curl -L https://pnpm.js.org/pnpm.js | node - add --global pnpm
+        pnpm install --frozen-lockfile
 
   unit_test: &unit_test
     steps:
       - checkout
       - *install_dependencies_step
       - run:
           name: tests
-          command: 'npm run test:ci'
+          command: 'pnpm test:ci'
           environment:
             JEST_JUNIT_OUTPUT: 'reports/junit/js-test-results.xml'
       - run:
           name: lint
-          command: 'npm run lint'
+          command: 'pnpm lint'
       - store_test_results:
           path: reports/junit
       - store_artifacts:
@@ -43,7 +45,7 @@ jobs:
       - *install_dependencies_step
       - run:
           name: Publish package
-          command: 'npm ci && npm run build && npx semantic-release'
+          command: 'pnpm build && npx semantic-release'
 
 workflows:
   version: 2

.eslintignore

@@ -0,0 +1 @@
+/dist

.eslintrc

@@ -3,6 +3,8 @@
     "eslint:recommended",
     "plugin:@typescript-eslint/recommended",
     "plugin:jest/all",
+    "plugin:import/recommended",
+    "plugin:import/typescript",
     "prettier"
   ],
   "env": {
@@ -13,7 +15,7 @@
   "parserOptions": {
     "sourceType": "module"
   },
-  "plugins": ["prettier", "jest", "@typescript-eslint/eslint-plugin"],
+  "plugins": ["prettier", "jest", "@typescript-eslint/eslint-plugin", "import"],
   "rules": {
     "jest/require-hook": "off",
     "no-unused-vars": "off",