Auto merge of #12559 - jdm:parallel-codegen, r=Ms2ger
Auto merge of #12559 - jdm:parallel-codegen, r=Ms2ger

Generate multiple DOM bindings in parallel.

Reduces the time for a build after touching `CodegenRust.py` by 58s for me.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12529 (github issue number if applicable).
- [X] These changes do not require tests because it's a build performance optimization

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