Fix the #[event(start)] regression from v0.8

[jakubadamw]

Fixes a regression from e363ba6#diff-5fbccf7aad8fecebdb79b59bcdf993ad2236c34b9360b7f6cc2862374497a610L244-R234 that made any use of #[event(start)] fail to compile unless the user crate had a direct dependency on wasm-bindgen.

Closes #973.

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 2 untouched benchmarks

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_{Comparing jakubadamw:issue-973 (98d14ae) with main (3dfe90e)}

[guybedford]

Thanks so much!