Register a protocol handler for matrix URIs

[anyone00]

Describe the problem

Add support for opening matrix URIs when clicked in the browser or the desktop application.

matrix-org/matrix-spec-proposals#2312
matrix-org/matrix.to#234

Documentation: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/registerProtocolHandler

Describe the solution you'd like

When clicking on the matrix URI like (matrix:u/test:matrix.org) the user should be able to set cinny as the protocol handler.

[aine-etke]

accidentally created duplicate #2333

[CrazyNicc]

internal matrix.to handling is starting to get deprecated now: matrix-org/matrix-spec-proposals#4487