emit-tsd: support multi-value-return wasm functions instead of asserting

[guybedford]

A wasm function returning multiple values trips a hard assertion in create_tsd and aborts the whole --emit-tsd pass:

assert len(functype.returns) <= 1, 'One return type only supported'

Multi-value return is standardized wasm and shows up in real toolchains — most notably wasm-bindgen, which emits (i32, i32) ptr+len pairs for any Rust export returning String, Vec<u8>, Result<T, E>, etc. on wasm32-unknown-emscripten. The wasm is valid; only the TSD pass crashes.

Replace the assert with the natural translation: a wasm function returning (i32, i32) becomes a TS function returning [number, number]. Empty- and single-return paths are unchanged.

The raw tuple isn't always what consumers ultimately want (wasm-bindgen's JS glue wraps these into a single JS value), but emitting something lets the .d.ts parse and lets downstream .d.ts mergers intersect with more precise types.

Test: test/other/test_emit_tsd_multivalue.c compiles a struct-returning function with -mmultivalue -Xclang -target-abi -Xclang experimental-mv and asserts the generated .d.ts contains the tuple type. Without the fix the new test reproduces the original AssertionError; with it, all existing --emit-tsd tests continue to pass.

[guybedford]

Closing in favour of #26966.