Express a set of string patterns which could be resolved into URLs

Currently clients resolve urls that look like URLs, but there are times when you want to resolve things like MSC numbers or other string patterns which may have a meaning in a subset of rooms. For example, if someone says MSC1234 in the `#matrix-spec` room then I'd like my client to linkify the text to be `[MSC1234](https://github.com/matrix-org/matrix-doc/issues/1234)`.

This could probably be expressed in a state event(s) and would be opt-in for clients, but would reduce the amount of noise created by bots trying to be helpful by spitting links underneath messages.

Start by reading the issue's examples of MSC1234 resolution and its suggestion of opt-in state events for room-specific patterns. No file or test is named; the work is done only after the protocol representation, client opt-in behavior, and URL resolution expectations are agreed and documented.

Written by the indexing model from the issue text.