[w3c/manifest] Adding protocol handlers (#972)

Closes #863 (#969)

This is an update that brings the protocol_handlers member addition up to date with the new structure of the specification (as stated in #969, it has been reworked since the previous effort was based on a very old version of the manifest spec).

cc @fabiorocha @aarongustafson  

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/w3c/manifest/pull/972.html" title="Last updated on Apr 26, 2021, 9:37 AM UTC (da610bd)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/972/61fdfe4...da610bd.html" title="Last updated on Apr 26, 2021, 9:37 AM UTC (da610bd)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/972

-- Commit Summary --

  * adds protocol_handlers member
  * Update index.mhtml (adds protocol_handlers)

-- File Changes --

    M index.html (206)

-- Patch Links --

https://github.com/w3c/manifest/pull/972.patch
https://github.com/w3c/manifest/pull/972.diff

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