Auto merge of #13729 - cynicaldevil:readAsArrayBuffer, r=Ms2ger

Implemented FileReader::readAsArrayBuffer

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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 #12555

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- [X] There are tests for these changes

There's still some small issues, but I suppose most of the work is done:
- test-tidy mentions a `method declared in webidl is missing a comment with a specification link` for the `getResult` method.
- I get an 'unused code' warning for code present in `UnionTypes.rs`, which is auto-generated.

Passing tests:
- [x]  `FileAPI/reading-data-section/filereader_result.html`
- [x]  `FileAPI/reading-data-section/filereader_readAsArrayBuffer.html`
- [x]  `FileAPI/idlharness.html`
- [ ]  `FileAPI/reading-data-section/FileReader-multiple-reads.html`

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