Add support for multiple `start` functions

[JoakoI98]

Summary

This PR adds the ability to define multiple start functions in a wasm-bindgen project by introducing a new multiple-start feature flag.

Motivation

When building WebAssembly modules that need to initialize global (shared) registries at module startup, there's often a need to run multiple initialization functions before user code executes. For example, if you have multiple crates or modules that each need to register themselves into a global registry, each requires its own start function to populate that registry.

Previously, wasm-bindgen would error with "cannot specify two 'start' functions" when multiple start functions were detected. This limitation made it difficult to compose modules that each have their own initialization requirements.

Implementation

The solution introduces a StartFunctionsHandler struct that acts as a coordinator for multiple start functions:

1. Wrapper Function Creation: When the first start function is encountered, a wrapper function named __wbindgen_start_wrapper is created. This wrapper becomes the "module's official" start function.

2. Aggregating Start Functions: Each subsequent start function is appended to the wrapper function's body via a call instruction. This ensures all start functions are executed in the order they were discovered.

3. Respecting Existing Transforms: The implementation preserves the existing behavior where start functions from thread/externref transforms run first (before user code), maintaining backward compatibility.

4. Cfg-Gated: The feature is opt-in via the wasm_bindgen_allow_multiple_start_functions cfg flag to avoid any breaking changes for existing users.

Changes

• crates/cli-support/src/wit/mod.rs: Added StartFunctionsHandler struct and refactored add_start_function() to support multiple start functions when the cfg flag is enabled

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #4842 will not alter performance

_{Comparing JoakoI98:multiple-start-functions (bd82afc) with main (3ef6481)}

Summary

✅ 4 untouched

[guybedford]

I'm not sure how I feel about this - is it not possible to define a start function that simply calls the other start functions? What inhibits doing this at this level that you'd need something upstream here?