[w3c/manifest] Adding protocolhandlers (#969)

Closes #863

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

* Breaks existing normative behavior (please add label "breaking")
* [x] Adds new normative recommendations or optional items
* [x] Makes only editorial changes (only changes informative sections, or changes normative sections without changing behavior)
* Is a "chore" (metadata, formatting, fixing warnings, etc).

Implementation commitment (delete if not making normative changes):

* [ ] Safari ([link to issue](https://bugs.webkit.org/show_bug.cgi?id=211401))
* [x] Chrome ([link to issue](https://bugs.chromium.org/p/chromium/issues/detail?id=1019239))
* [ ] Firefox ([link to issue](https://bugzilla.mozilla.org/show_bug.cgi?id=1635272))
* [x] Edge ([public signal](https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/URLProtocolHandler/explainer.md))

If change is normative, and it adds or changes a member:

* [ ] [updated JSON schema](https://github.com/SchemaStore/schemastore/blob/master/src/schemas/json/web-manifest.json)

Commit message:

Adds "protocol_handlers" member to the manifest spec, referencing the process that navigator.registerProtocolHandler uses.

contributors:
+@fabiorocha

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

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/969

-- Commit Summary --

  * Add protocol handlers to the explainer and spec
  * adds reference to registering handlers in HTML spec
  * Tidy up and added references

-- File Changes --

    M explainer.md (20)
    M index.html (201)

-- Patch Links --

https://github.com/w3c/manifest/pull/969.patch
https://github.com/w3c/manifest/pull/969.diff

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