Support JS libraries self-registering their exports via EXPORTED_FUNCTIONS

[guybedford]

This implements support for JS libraries adding their own symbols to EXPORTED_FUNCTIONS at library load time, split out from #27208 as requested in review there.

JS library code can already mutate the compile-time EXPORTED_FUNCTIONS set (it is exposed to library code as a Set), which under MODULARIZE=instance causes jsifier to emit the symbol with an export declaration. What was missing is the linker knowing about those additions:

• The final EXPORTED_FUNCTIONS set is now forwarded back from the JS compiler in FORWARDED_DATA, so the linker can derive which JS library symbols were exported.
• The WASM_ESM_INTEGRATION wrapper re-exports those symbols from the support module.

This is a requirement for wasm-bindgen's binding layers that define a public JS API surface distinct from the wasm export names (e.g. exported JS classes and namespaces), registering it from their generated JS library:

EXPORTED_FUNCTIONS.add('Counter');
addToLibrary({ $Counter: class Counter { /* ... */ } });

Test coverage is included in test_jslib.py for both MODULARIZE=instance and WASM_ESM_INTEGRATION output modes, where the WASM_ESM_INTEGRATION variant fails without this change.

Made with AI assistance under my review