fix(fetch): Correctly decode `multipart/form-data` names and filenames (#19145)

Currently the `multipart/form-data` parser in
`Request.prototype.formData` and `Response.prototype.formData` decodes
non-ASCII filenames incorrectly, as if they were encoded in Latin-1
rather than UTF-8. This happens because the header section of each
`multipart/form-data` entry is decoded as Latin-1 in order to be parsed
with `Headers`, which only allows `ByteString`s, but the names and
filenames are never decoded correctly. This PR fixes this as a
post-processing step.

Note that the `multipart/form-data` parsing for this APIs in the Fetch
spec is very much underspecified, and it does not specify that names and
filenames must be decoded as UTF-8. However, it does require that the
bodies of non-`File` entries are decoded as UTF-8, and in browsers,
names and filenames always use the same encoding as the body.

Closes #19142.

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