Auto merge of #15590 - gregkatz:webidl-enum-as-str, r=Ms2ger

Adds an `as_str()` method to WebIDL enums to hide slice of strings

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This PR adds an `as_str()` method to WebIDL enums to hide slice of strings from callers. It uses the new method in two places. It also eliminates a to_string() call that seemed unnecessary to 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 #15586 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because github issue says no tests needed.

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