Add matrix to the safelisted schemes list

The matrix scheme has recently been finalized and is documented at [1].

Matrix is an open and secure communication protocol, designed for an
open federation of servers, much like e-mail or xmpp.

The Matrix.org foundation [2] is the custodian of the matrix specification,
including the scheme linked above. As a neutral party, it will ensure
that future iteration of the matrix scheme will remain in line with its
stated goals.

The federated (possibly later distributed) nature of the Matrix network calls
for an URI scheme, which multiple software programs already implement
(namely, Matrix clients Fluffychat, Quaternion, and Nheko among others).
Multiple web clients (Element, Hydrogen, Fluffychat again) have
expressed their interest in registering as matrix protocol handlers.

This commits adds the provisional matrix scheme, as provisionally registered
at [3].
The Matrix network currently hosts a few million accounts[4], as well as
being the instant communication platform of choice for the KDE and Mozilla
communities.

[1]: matrix-org/matrix-spec-proposals#2312
[2]: https://matrix.org/foundation/
[3]: https://www.iana.org/assignments/uri-schemes/prov/matrix
[4]: https://matrix.org/faq/#what-is-the-current-project-status

source

@@ -94935,6 +94935,7 @@ interface <dfn>Navigator</dfn> {
      <li><code data-x="">ircs</code></li>
      <li><code data-x="">magnet</code></li>
      <li><code data-x="">mailto</code></li>
+     <li><code data-x="">matrix</code></li>
      <li><code data-x="">mms</code></li>
      <li><code data-x="">news</code></li>
      <li><code data-x="">nntp</code></li>
@@ -124241,6 +124242,7 @@ INSERT INTERFACES HERE
   Mattias Waldau,
   Max Romantschuk,
   Maxim Tsoy, <!-- muodov on GitHub -->
+  Mayeul Cantan,
   Menachem Salomon,
   Menno van Slooten,
   Micah Dubinko,