Add XHR support for FormData and related fixes
+ Update IDL for `FormData`, and related fixes
+ Reuse `htmlformelement::FormDatum` for `formdata.rs` for a uniform representation, and related fixes
+ Add support for `FormData` branch of XHR `BodyInit`.
+ Fixes a lot of related WPT tests
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes OR
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