Add unstable_lazilyDefineModules serializer (switch-bundle output)

[robhogan]

Summary:
Ports the metro-buck "plain bundle with switch" output format to OSS Metro, behind a new opt-in serializer.unstable_lazilyDefineModules.

baseJSBundleWithLazyModuleDefinition emits the graph's modules inside a single segment definer:

__registerSegment(0, function (moduleId) { switch (...) })

...so each module's __d(...) runs lazily on first require, rather than eagerly at startup. Polyfills/runtime (pre-modules) and run-module calls stay eager. This mirrors plain-bundle-with-switch.js in the metro-buck worker, built on the same OSS wrapModule primitive, but parameterised on OSS Module/graph types.

Because the switch wraps all modules into one function, the flat index-map path (fromRawMappingsIndexed, which assumes the plain top-level __d layout) would misalign, so code and source map are assembled together via metro-source-map's BundleBuilder, which offsets each module's map section by its actual position in the output.

The structured graph is untouched, so deltas and HMR (addressed per-module via hmrJSBundle) are unaffected: an HMR update remains a top-level __d(...) that shadows the switch branch, and the runtime materialises a not-yet-required module on demand via its segment definer.

This diff only adds the serializer + config. Wiring into Server (dev + prod) follows in the next diff.

Changelog:

 - **[Experimental]**: Add serializer.unstable_lazyDefineModules for deferred __d calls

Reviewed By: huntie

Differential Revision: D113676086

[meta-codesync]

@robhogan has exported this pull request. If you are a Meta employee, you can view the originating Diff in D113676086.

[meta-codesync]

This pull request has been merged in 8d92267.