Fix wasm-bindgen-test compatibility with doctests in Node.js

[drewcrawford]

This commit addresses crashes when running wasm-bindgen-test in doctests, which execute in Node.js environments with specific limitations.

Problem:
When running doctests with wasm-bindgen-test, async code using spawn_local() would crash with "not a shared typed array" error. This occurred because:

1. Doctests run in Node.js, not browsers
2. Node.js may not have SharedArrayBuffer enabled by default
3. Even with atomics compiled in, the runtime memory might not be shared

Solution:

1. SharedArrayBuffer detection: Before using Atomics.waitAsync(), we now check if the memory buffer is actually a SharedArrayBuffer. If not, we gracefully fall back to the polyfill implementation.

2. Worker availability handling: In Node.js doctests, Worker may not be available or may behave differently. The polyfill now:

   • Checks for Worker constructor availability before attempting to use it
   • Falls back to setTimeout-based polling when Worker is unavailable
   • Implements exponential backoff (1ms → 2ms → 4ms... up to 100ms)
   • Polls up to 10 times before timing out

Design decisions:

• The SharedArrayBuffer check uses inline JS to properly detect the type, as this can't be reliably done from Rust alone
• The polling fallback provides basic async behavior without Worker deps, making doctests functional even in restricted environments
• The implementation prioritizes compatibility over performance in fallback scenarios, as doctests are primarily for documentation/testing

This enables wasm-bindgen-test to work reliably in doctest environments while maintaining full functionality in production browser contexts.