feat: add CJS build output for dual format (#518)

Ideally we want to use ESM, but the reality is that
@fluentui/react-theme (and other Fluent packages) ESM lacks Node.js
compliance (missing .js extensions) - it works in bundlers but not in
Node.js.

This means downstream tools are forced to use a CJS bundler, which can
`require` Fluent UI from `node_modules` because Fluent has CJS exports
as a fallback.

However, this package doesn't provide CJS, so such a downstream bundler
cannot `require` it and must bundle our source into its own dist instead.

By adding CJS here, downstream libraries can externalize us from
node_modules, reducing their bundle sizes. Otherwise we become a nested
dependency.

- Add build:cjs script and lib-commonjs output
- Update exports with require condition
- Remove "type": "module" for dual-format semantics

Author: mtqperson

biome.json

@@ -10,11 +10,11 @@
       "**/*.ts",
       "**/*.tsx"
     ],
-    "ignore": ["lib/", ".vscode/"]
+    "ignore": ["lib/", "lib-commonjs/", ".vscode/"]
   },
   "organizeImports": {
     "enabled": true,
-    "ignore": ["generated/", ".vscode/"]
+    "ignore": ["generated/", "lib-commonjs/", ".vscode/"]
   },
   "formatter": {
     "enabled": true,
@@ -23,7 +23,7 @@
   },
   "linter": {
     "enabled": true,
-    "ignore": ["generated/", ".vscode/"],
+    "ignore": ["generated/", "lib-commonjs/", ".vscode/"],
     "rules": {
       "recommended": false,
       "correctness": {

theme/lib-commonjs/index.js

@@ -10,20 +10,21 @@
   },
   "license": "MIT",
   "author": "Axis Communications AB",
-  "type": "module",
   "exports": {
     ".": {
       "types": "./lib/index.d.ts",
-      "import": "./lib/index.js"
+      "import": "./lib/index.js",
+      "require": "./lib-commonjs/index.js"
     }
   },
   "bin": {
     "fluent-theme-tokens": "./lib/tokens.cjs"
   },
-  "files": ["lib"],
+  "files": ["lib", "lib-commonjs"],
   "scripts": {
-    "build": "pnpm build:types && pnpm build:esm && pnpm build:cli",
+    "build": "pnpm build:types && pnpm build:esm && pnpm build:cjs && pnpm build:cli",
     "build:cli": "esbuild --format=cjs --bundle --platform=node --packages=external --outfile=lib/tokens.cjs src/tokens/build.ts",
+    "build:cjs": "esbuild --format=cjs --bundle --sourcemap --packages=external --outdir=lib-commonjs src/index.ts",
     "build:esm": "esbuild --format=esm --bundle --sourcemap --packages=external --outdir=lib src/index.ts",
     "build:types": "tsc -p tsconfig.build.json",
     "check:unused-deps": "depcheck . --config=depcheck.yml",