chore(build): add devtools-formatter-worker.ts bundle

[szuend]

The devtools-formatter-worker is a separate entrypoint from the MCP entrypoint. It is identical to the upstream front_end/entrypoints/formatter_worker/formatter_worker-entrypoint.js and a required part to make source mapping, formatting and other AST-based DevTools features work.

The implementation consists of 3 parts:

1. A trivial devtools-formatter-worker.ts file that imports the DevTools entrypoint.
2. A new bundle output, bundling everything that needs to run in the worker.
3. A call that initializes the FormatterWorkerPool with the custom entrypoint. Otherwise we'd use the unbundled entrypoint from chrome-devtools-frontend not available to us at runtime.

Note that we could improve code splitting in the future as some things like core/common are now bundled twice.