maint/dev ~ add Rollup support (for generation of ESM with correct ex…

…tensions)

- TypeScript fails to correctly generate an ESM module with correct file extensions
  - ref: <microsoft/TypeScript#18442 (comment)>
  - ref: <microsoft/TypeScript#18442>
  - ref: <microsoft/TypeScript#37582>
  - ref: <microsoft/TypeScript#35148>

.rollup.config.js

@@ -0,0 +1,19 @@
+// ref: <https://devhints.io/rollup>
+// setup: `npm i rollup @rollup/plugin-typescript` or `npm i rollup rollup-plugin-typescript2` (for visible TS error output)
+
+// import typescript from '@rollup/plugin-typescript';
+import typescript from 'rollup-plugin-typescript2';
+
+export default [
+	// ES module build (replaces broken basic TypeScript compilation)
+	// * ref: <https://github.com/microsoft/TypeScript/issues/18442> , <https://github.com/alshdavid/rxjs/blob/main/rollup.config.js#L10>
+	// * ref: <https://github.com/microsoft/TypeScript/pull/35148>
+	// * ref: <https://github.com/microsoft/TypeScript/issues/37582>
+	{
+		preserveModules: true,
+		input: ['src/index.ts'],
+		external: ['path', 'os', 'os-paths'],
+		output: [{ exports: 'auto', dir: 'build/esm', format: 'esm', entryFileNames: '[name].mjs' }],
+		plugins: [typescript({ tsconfig: './tsconfig/tsconfig.esm.json' })],
+	},
+];