[w3c/manifest] First pass as adding a Client Hint for Display Mode (#977)

Closes #954

This change (choose at least one, delete ones that don't apply):

* Adds new normative requirements

Implementation commitment (delete if not making normative changes):

* [ ] Safari (link to issue)
* [ ] Chrome (link to issue)
* [ ] Firefox (link to issue)
* [ ] Edge (public signal)

Commit message:

Adds a Client Hint to reveal the Display Mode being used by the user agent.

Person merging, please make sure that commits are squashed with one of the following as a commit message prefix:

* chore:
* editorial:
* BREAKING CHANGE:
* And use none if it's a normative change


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<a href="https://pr-preview.s3.amazonaws.com/aarongustafson/manifest/pull/977.html" title="Last updated on May 7, 2021, 10:45 PM UTC (5714fb0)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/977/5ba7e51...aarongustafson:5714fb0.html" title="Last updated on May 7, 2021, 10:45 PM UTC (5714fb0)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/977

-- Commit Summary --

  * Editorial: First pass as adding a Client Hint for Display Mode

-- File Changes --

    M index.html (42)

-- Patch Links --

https://github.com/w3c/manifest/pull/977.patch
https://github.com/w3c/manifest/pull/977.diff

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