Auto merge of #12634 - malisas:malisa-headersAPI, r=jdm

Headers API constructor and methods

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This PR fills out the constructor and the delete, get, has, and set methods for the Headers API. Addresses issue #11897 .

The PR also rewrites the append method to support `hyper::header::Headers`'s HashMap `insert` method, which overwrites entries instead of appending.
As a result of this, for a given header name there is at most one value in the inner "header list"/HashMap. Multiple values for the same name are comma-delimited.

There are still a few TODOs:
- Support `OpenEndedDictionary<ByteString>`  as a possible `HeadersInit` value. [OpenEndedDictionary<T> is a future IDL construct.](https://fetch.spec.whatwg.org/#headers-class)
- Support `iterable<ByteString, ByteString>`. Related issue: #12628
- Values are comma-delimited, except for values with the name `set-cookie`, which are newline-delimited. This is because values for `set-cookie` are [allowed to contain](https://tools.ietf.org/html/rfc7230#section-3.2.2) inner commas. This violates the [spec](https://fetch.spec.whatwg.org/#concept-header-value-combined).
- The TODOs from PR #12467 regarding value parsing also still need to be resolved.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because tests for the Headers API already exists, but this commit does not implement the interface fully. The tests will fail.

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