[lld] [lld][WebAssembly] Add an `--initial-heap` option (PR #75594)

[Sam Clegg via llvm-commits]

https://github.com/sbc100 approved this pull request.

Thanks for taking the time to work on this.    

This looks really good.   I can't remember that last time I got a change this big that didn't warrant some feedback!

https://github.com/llvm/llvm-project/pull/75594