[whatwg/fetch] Add process model processing for the "Priority" header. (PR #1757)

- [x] At least two implementers are interested (and none opposed):
   * Chrome
   * Firefox
   * See discussion in #1718
- [ ] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
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- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: …
   * Gecko: …
   * WebKit: …
   * Deno (not for CORS changes): …
- [ ] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: …
- [x] The top of this comment includes a [clear commit message](https://github.com/whatwg/meta/blob/main/COMMITTING.md) to use. <!-- If you created this PR from a single commit, Github copied its message. Otherwise, you need to add a commit message yourself. -->

(See [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)

This adds the processing for the `Priority` HTTP header from HTTP Extensible Priorities ([RFC 9218](https://datatracker.ietf.org/doc/rfc9218/)).

The actual priority values are implementation-specific but this change makes it clear that an existing value should not be overwritten if the header is already present.

Fix #1718


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  https://github.com/whatwg/fetch/pull/1757

-- Commit Summary --

  * Add process model processing for the "Priority" header.

-- File Changes --

    M fetch.bs (6)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1757.patch
https://github.com/whatwg/fetch/pull/1757.diff

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