On expanding the valid scheme set

[SoniEx2]

Currently, the valid scheme set only supports one or more ascii-lowercase.

This rules out hypothetical schemes like web+activitypub-post:?summary=Post%20created%20with%20protocol%20handlers&message=Hello%20from%20protocol%20handlers!. It is currently unclear whether we want to support these, so this discussion is intended to track possible changes in this problem space.