Unify wasm-bindgen output under -sWASM_BINDGEN, add -sWASM_BINDGEN=auto

[guybedford]

This unifies wasm-bindgen output generation under a single -sWASM_BINDGEN path and adds -sWASM_BINDGEN=auto, building on the wasm-bindgen side change in wasm-bindgen/wasm-bindgen#5210 (shipped in 0.2.126). It supersedes #27179.

Previously -sWASM_BINDGEN was effectively a staticlib-only flow where emcc had to discover and own the export set itself. With #5210, wasm-bindgen hoists its clean exported API (functions, classes, enums) into top-level library symbols that self-register into EXPORTED_FUNCTIONS via its --js-library. The user-facing API now comes from wasm-bindgen's own library in every flow, so emscripten no longer needs to guess or own exports — it just surfaces what wasm-bindgen registered. That lets one -sWASM_BINDGEN path serve both whether cargo/rustc drives the link (bin crate, emcc as the linker, rustc supplies -sEXPORTED_FUNCTIONS) or emcc drives it (staticlib, exports discovered locally).

The model for a public export is the union across both systems: wasm-bindgen's self-registered API, plus emscripten's own EMSCRIPTEN_KEEPALIVE exports — minus wasm-bindgen's internal expansion glue, which must not spill into the public surface.

What's implemented:

• The exports the wasm-bindgen expansion reaches by name — the supplied EXPORTED_FUNCTIONS (method shims, the __wbindgen_* runtime, the marker, main) plus anything its expansion adds — are captured as internal glue and kept off every export layer: the ESM wrapper (user_requested_exports), the factory Module attachment (EXPORTED_FUNCTIONS, via should_export), and the keepalive pass in finalize_wasm. main still runs automatically on init; _main isn't surfaced.
• A genuine EMSCRIPTEN_KEEPALIVE C/C++ export is not in that internal set and remains surfaced, so a hand-written native export composes with wasm-bindgen's API in the same module.
• Placeholder symbols wasm-bindgen consumes (__wbindgen_describe*, __externref_*, ...) are stripped so they aren't reported as undefined exports.
• nm-based export discovery only runs for explicit -sWASM_BINDGEN when no driver supplied EXPORTED_FUNCTIONS; the rustc-driven link already lists them exactly.
• Imported JS is wired through: library_bindgen.extern-pre.js is fed as extern-pre-js and the snippets/ dir is copied next to the output so relative imports resolve.
• The WASM_ESM_INTEGRATION wrapper re-exports the JS library symbols that were exported (MODULARIZE=instance), and provides wasmExports via a namespace import of the wasm so by-name export access works.

-sWASM_BINDGEN=auto (replacing #27179's implicit marker detection, per review feedback there): runs wasm-bindgen only when the linked wasm carries wasm-bindgen's __wasm_bindgen_emscripten_marker custom section — how cargo/rustc opts in when driving emcc as the linker. Otherwise it's a no-op and wasm-bindgen need not be installed, so rustc can always pass it via -Clink-arg without affecting ordinary builds.

if settings.WASM_BINDGEN == 'auto':
    settings.WASM_BINDGEN = 1 if building.is_wasm_bindgen_module(in_wasm) else 0

Both output modes then expose only the clean API (e.g. a Greeter class).

Testing:

• End-to-end test parameterized over the ESM (-sWASM_ESM_INTEGRATION) and factory (-sMODULARIZE -sEXPORT_ES6) output modes, built via cargo build with -sWASM_BINDGEN=auto (marker auto-detected), asserting the clean Greeter API works, main runs on init, and none of the raw wasm exports leak.
• A no-marker test asserting -sWASM_BINDGEN=auto is a no-op for an ordinary hello_world.c build (doesn't require wasm-bindgen installed).
• The staticlib integration test is extended to assert an EMSCRIPTEN_KEEPALIVE native export survives alongside the wasm-bindgen API (fails under a blanket export wipe).
• CI installs a pinned wasm-bindgen-cli alongside rust so the flow is always exercised.

Not in scope (follow-up): preserving human-supplied EXPORTED_FUNCTIONS through wasm-bindgen linkage (the rustc-supplied set is currently indistinguishable from glue, so explicit exports aren't surfaced).