Add typecsript sample rollup config + exploration of different plugins' side-effects

packages/addon-dev/sample-typescript-babel.config.cjs

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+'use strict';
+
+const { resolve } = require;
+
+module.exports = {
+  presets: [resolve('@babel/preset-typescript')],
+  plugins: [
+    [
+      resolve('@babel/plugin-transform-typescript'),
+      {
+        allowDeclareFields: true,
+        onlyRemoveTypeImports: true,
+        // Default enums are IIFEs
+        optimizeConstEnums: true,
+      },
+    ],
+    [
+      resolve('@babel/plugin-proposal-decorators'),
+      {
+        // The stage 1 implementation
+        legacy: true,
+      },
+    ],
+    [resolve('@babel/plugin-proposal-class-properties')],
+    resolve('@embroider/addon-dev/template-colocation-plugin'),
+  ],
+};

packages/addon-dev/sample-typescript-rollup.config.mjs

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+import ts from 'rollup-plugin-ts';
+import { defineConfig } from 'rollup';
+
+import { Addon } from '@embroider/addon-dev/rollup';
+
+const addon = new Addon({
+  srcDir: 'src',
+  destDir: 'dist',
+});
+
+export default defineConfig({
+  // https://github.com/rollup/rollup/issues/1828
+  watch: {
+    chokidar: {
+      usePolling: true,
+    },
+  },
+  output: {
+    ...addon.output(),
+    sourcemap: true,
+    // Until a bug with the ember-cli-htmlbars-removal transform is fixed, you
+    // may need
+    // hoistTransitiveImports: false
+    // specified here as well
+  },
+  plugins: [
+    // These are the modules that users should be able to import from your
+    // addon. Anything not listed here may get optimized away.
+    addon.publicEntrypoints([
+      'index.ts',
+      'components/**/*.ts',
+      'helpers/**/*.ts',
+      'modifiers/**/*.ts',
+      'services/**/*.ts',
+      'test-support/**/*.ts',
+    ]),
+
+    // These are the modules that should get reexported into the traditional
+    // "app" tree. Things in here should also be in publicEntrypoints above, but
+    // not everything in publicEntrypoints necessarily needs to go here.
+    addon.appReexports([
+      'index.js',
+      'components/**/*.js',
+      'helpers/**/*.js',
+      'modifiers/**/*.js',
+      'services/**/*.js',
+      'test-support/**/*.js',
+    ]),
+    // This babel config should *not* apply presets or compile away ES modules.
+    // It exists only to provide development niceties for you, like automatic
+    // template colocation.
+    // See `babel.config.json` for the actual Babel configuration!
+    ts({
+      // can be changed to swc or other transpilers later
+      // but we need the ember plugins converted first
+      // (template compilation and co-location)
+      transpiler: 'babel',
+      // without the browserlist, recently landed JS features are polyfilled.
+      // It should be primarily up to the consuming app whether or not they need to
+      // polyfill whatever JS features.
+      browserslist: ['last 2 firefox versions', 'last 2 chrome versions'],
+      tsconfig: {
+        // when `hooks` is specified, fileName is required
+        fileName: 'tsconfig.json',
+        hook: (config) => ({
+          ...config,
+          declaration: true,
+          declarationMap: true,
+          // See: https://devblogs.microsoft.com/typescript/announcing-typescript-4-5/#beta-delta
+          // Allows us to use `exports` to define types per export
+          // However, we can't use that feature until the minimum supported TS is 4.7+
+          declarationDir: './dist',
+        }),
+      },
+    }),
+
+    // Follow the V2 Addon rules about dependencies. Your code can import from
+    // `dependencies` and `peerDependencies` as well as standard Ember-provided
+    // package names.
+    addon.dependencies(),
+
+    // Ensure that standalone .hbs files are properly integrated as Javascript.
+    addon.hbs(),
+
+    // addons are allowed to contain imports of .css files, which we want rollup
+    // to leave alone and keep in the published output.
+    addon.keepAssets(['**/*.css']),
+
+    addon.clean(),
+  ],
+});