feat: external bundles

[zamotany]

Adds a way to use pre-built (DLL) bundle when using multi-bundle command.

TODO

• add support for bundlePath and manifestPath to multi-bundle command
• add support for bundlePath and manifestPath to `packager server
• serve source maps for pre-built bundle from packager server
• add copyBundle option to automatically copy files to target directory
• add integration test
• monorepo fixture
• monorepo integration test
• add documentation
• properly handle and resolve assets from pre-built bundle

[thymikee]

Looking good!