Migrate to ES Modules with Rollup

[Copilot]

Migrates the build toolchain from CommonJS + @vercel/ncc to ES Modules + Rollup, following shogo82148/typescript-action#433.

Build

• Replace @vercel/ncc with rollup + @rollup/plugin-{commonjs,node-resolve,typescript,json}
• Output as CJS (dist/index.cjs) — required because transitive deps (@actions/cache, @actions/tool-cache, undici, etc.) contain dynamic require() calls that rollup can't statically convert; .cjs is always loaded as CommonJS by Node.js regardless of "type": "module"
• action.yml updated to main: "dist/index.cjs"

TypeScript / module config

• Add tsconfig.base.json: NodeNext module resolution, ES2022 target
• Add tsconfig.eslint.json: extends base with isolatedModules: true, noEmit: true — used by Jest and ESLint
• tsconfig.json now extends base, outputs to ./dist
• All local imports use .js extensions (required by NodeNext)

Source

• src/index.ts is the new action entrypoint; calls run() exported from src/main.ts
• src/main.ts exports run() instead of invoking it directly

Tests

• jest.config.js converted to ESM (export default), useESM: true, ts-jest-resolver
• cross-env added so NODE_OPTIONS=--experimental-vm-modules works on Windows
• __dirname usage in __test__/installer.test.ts replaced with import.meta.url

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