Auto merge of #20431 - talklittle:issues-13234-5600-squashed, r=jdm

TextDecoder: streaming decode, ignoreBOM

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Implement streaming decode and ignoreBOM flag for TextDecoder.

https://encoding.spec.whatwg.org/#dom-textdecoder-decode

https://encoding.spec.whatwg.org/#dom-textdecoder-ignorebom

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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 #13234 (github issue number if applicable).
- [x] These changes fix #5600 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because the wpt tests are used for testing:

* /encoding/textdecoder-fatal-streaming.html
* /encoding/textdecoder-streaming.html
* /encoding/textdecoder-ignorebom.html

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