feat: Add TypeScript type definitions

Also add @types/prettier as dependency.

index.d.ts

@@ -0,0 +1,28 @@
+/**
+ * @file Type definition for rollup-plugin-prettier
+ */
+
+import type { Options as PrettierOptions } from "prettier";
+import type { Plugin } from "rollup";
+
+declare namespace prettier {
+  interface Options extends PrettierOptions {
+    /**
+     * Directory to look for a Prettier config file.
+     *
+     * If omitted, defaults to `process.cwd()`.
+     */
+    cwd?: string;
+    /**
+     * Whether to generate a sourcemap.
+     *
+     * Note: This may take some time because rollup-plugin-prettier diffs the
+     * output to manually generate a sourcemap.
+     */
+    sourcemap?: boolean;
+  }
+}
+
+declare function prettier(options?: prettier.Options): Plugin;
+
+export = prettier;

package.json

@@ -9,6 +9,7 @@
     "type": "git",
     "url": "git@github.com:mjeanroy/rollup-plugin-prettier.git"
   },
+  "types": "./index.d.ts",
   "scripts": {
     "clean": "gulp clean",
     "lint": "gulp lint",
@@ -31,6 +32,7 @@
     "rollup": "^1.0.0 || ^2.0.0"
   },
   "dependencies": {
+    "@types/prettier": "^1.0.0 || ^2.0.0",
     "diff": "5.0.0",
     "lodash.hasin": "4.5.2",
     "lodash.isempty": "4.4.0",