Auto merge of #17855 - asajeffrey:script-implement-more-stylepropertymaypreadonly, r=emilio

Implement getProperties for StylePropertyMapReadOnly

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Implement `getProperties` for `StylePropertyMapReadOnly`.

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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 #17579.
- [X] These changes do not require tests because the existing css-paint-api tests catch this (rather annoyingly, they all fail for different reasons, sigh)

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