Support decoding matrix URI scheme

Nobody has claimed this yet.

MSC2312: Matrix URI scheme proposal https://github.com/matrix-org/matrix-doc/pull/2312 is nearing completion.

As such, we can expect apps to progressively switch from `https://matrix.to` links to the `matrix:` scheme.

**Describe the solution you'd like**
In order to provide a smooth transition, as well as a fallback when no client is installed, I would like matrix.to to implement support for parsing `matrix:` URI as input, and work as usual with the decoded information:
- [ ] define a way for `matrix.to` to receive `matrix` URIs, it could be something like `https://matrix.to/#/url/%s` (percent-encoded of course)
- [ ] Implement parsing for it as specified in [MSC2312](https://github.com/matrix-org/matrix-doc/pull/2312)
- [ ] modify web browsers to add matrix.to as a default handler for these links
* For Firefox the magic happens [here](https://searchfox.org/mozilla-central/source/browser/locales/en-US/chrome/browser-region/region.properties) with the tests [there](https://searchfox.org/mozilla-central/source/uriloader/exthandler/tests/unit/test_handlerService_store.js#557)
* Unsure if Chromium has something similar
- [ ] ask visitors if they want to register matrix.to as a protocol handler for `matrix:` links (can also ease the transition)

**Describe alternatives you've considered**
Falling back to Element.io is another possibility that can be examined, I am not sure which one brings the best onboarding experience. Both are not exclusive.
However, Element.io is a lot less neutral, so it would make it a harder sell to browsers, does not educate users as well, and is already quite well emphasized in matrix.to.

**Additional Context**
Users might want to copy-paste a URL from the address bar? In that case, maybe rewrite them to regular matrix.to format?

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Firefox includes default handlers, as linked above, and from the following screenshot of a web browser that has never been on yahoo nor gmail:

Start by reading MSC2312 and the existing matrix.to URL handling; the issue does not name repository files. Review Firefox's browser/locales/en-US/chrome/browser-region/region.properties and uriloader/exthandler/tests/unit/test_handlerService_store.js references for protocol-handler context. Done would require an agreed scope covering matrix: parsing, browser fallback or registration, and compatibility with existing matrix.to links.

Written by the indexing model from the issue text.