Adds support for colons in directive names

[maximilliangeorge]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)
• If applicable, I’ve added docs and tests

Description of changes

This PR adds support for colons within a leaf directory name. This syntax is helpful to create namespaced methods, like so: ::hello:alice and ::hello:bob.

I needed this for a particular use case and wasn't planning on submitting a PR to the original repo. In honesty, it probably shouldn't be merged since it isn't according to the common spec. Still, I'm submitting it here for transparency and interesting conversations.

[wooorm]

I like this because JSX/XML/HTML also support a “namespace” (:svg:rect).
I dislike this because it is close to conflicting with gemoji shortcodes (see also this related fix) in the text variant (:wave:how are you?). It’s probably not common to use a word directly after a gemoji, but gemoji are very common, so I’d imagine it exists in the wild.

If a colon was supported, I think we shouldn’t allow them arbitrarily: :a::::::::::::::::b seems weird. Instead, I think I’d like them to work as “two” names, with a single colon in between, and that dashes or underscores cannot be used to end each name part (so :a-:b would not be ok).

[wooorm]

I needed this for a particular use case and wasn't planning on submitting a PR to the original repo. In honesty, it probably shouldn't be merged since it isn't according to the common spec. Still, I'm submitting it here for transparency and interesting conversations.

Coming back to this, I too have some doubts. How far colons should go, if they should go. So, closing for now. But, I do welcome hearing more use cases!