Add ESM build

Author: drwpow

.gitignore

@@ -128,3 +128,7 @@ dist
 .yarn/build-state.yml
 .yarn/install-state.gz
 .pnp.*
+
+# Generated files
+index.mjs
+index.d.mts

.npmignore

@@ -0,0 +1,5 @@
+.github
+.gitattributes
+scripts
+test
+**/*.test-d.ts

package.json

@@ -5,12 +5,27 @@
   "main": "index.js",
   "type": "commonjs",
   "types": "types/index.d.ts",
+  "exports": {
+    ".": {
+      "import": {
+        "default": "./index.mjs",
+        "types": "./types/index.d.mts"
+      },
+      "default": {
+        "default": "./index.js",
+        "types": "./types/index.d.ts"
+      }
+    },
+    "./*": "./*"
+  },
   "scripts": {
     "lint": "eslint",
     "lint:fix": "eslint --fix",
     "test:unit": "c8 --100 node --test",
     "test:typescript": "tsd",
-    "test": "npm run lint && npm run test:unit && npm run test:typescript"
+    "test": "npm run lint && npm run test:unit && npm run test:typescript",
+    "build": "node ./scripts/generate-esm.mjs",
+    "version": "npm run build"
   },
   "precommit": [
     "lint",

scripts/generate-esm.mjs

@@ -0,0 +1,40 @@
+import fs from 'node:fs'
+
+const CWD = new URL('../', import.meta.url)
+
+// list of all inputs/outputs, with replacements
+const SOURCE_FILES = [
+  {
+    source: './index.js',
+    types: './types/index.d.ts',
+    replacements: [
+      [/const\s+([^=]+)=\s*require\(([^)]+)\)/g, (_, spec, moduleName) => {
+        return `import ${spec.trim().replace(/:\s*/g, ' as ')} from ${moduleName.trim()}`
+      }],
+      [/module\.exports\s*=\s*({[^}]+})/, 'export $1'],
+    ]
+  }
+]
+
+// Build script
+for (const { source, types, replacements } of SOURCE_FILES) {
+  // replace
+  let output = fs.readFileSync(new URL(source, CWD), 'utf8')
+  for (const [search, replaceValue] of replacements) {
+    output = output.replace(search, replaceValue)
+  }
+
+  // verify
+  if (output.includes('require(')) {
+    throw new Error('Could not convert all require() statements')
+  }
+  if (output.includes('module.exports')) {
+    throw new Error('Could not convert module.exports statement')
+  }
+
+  // write source
+  fs.writeFileSync(new URL(source.replace(/\.js$/, '.mjs'), CWD), output)
+
+  // write types
+  fs.copyFileSync(new URL(types, CWD), new URL(types.replace(/\.d\.ts$/, '.d.mts'), CWD))
+}