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

[sbc100]

I think supporting something like this is a great idea but maybe we can bikeshed the design a little.

How about limiting this to symbols actually defined in the JS library itself using symbol attributes. Perhaps something like:

myLibrary = {
   foo: 42,
   foo__export: true,  // Export this symbol outside the module
   foo__alwasyInclude: true, // Include this symbol in the generated code, even if no other code depends on it.
}

I don't love the alwasyInclude name.. perhaps force?

In this scenario it would be possible to specify one of these without that other.

If you specified __export without __force then them symbol would be exported, but only if it was included for some reason.

If you specified __force without __export then the symbol would be included in the generated JS but not exported (similar to (similar to DEFAULT_LIBRARY_FUNCS_TO_INCLUDE).

I would hope that the information could all stay on the JS side, but if it needs to be plumbed back to python then it could then just be small extraExports list perhaps?