Rename/document special key in theme files

[ChrHorn]

There is a key special in theme files that is used for the fuzzy highlighting in the file/command picker

this key is

• undocumented
• has a misleading name
• might also be used as a treesitter grammar scope (not sure)

[archseer]

It's special because it's used for special syntax symbols like ? operator in rust. The fuzzy finder PR that implemented highlighting simply reused it because it had the right color.

[ChrHorn]

Makes sense #1635

In my opinion this should be a seperate entry, maybe default to bold?
Better to keep syntax highlighting and UI seperate.

[the-mikedavis]

I've been meaning to make this same issue 😀

I wouldn't mind keeping around special for things like Rust's ? but there are other cases where it makes less sense that I've come across:

• Elixir sigils
• markdown bullet points / lists
• matching for fuzzy highlighting

Some of these can be themed with existing scopes but others need some new scopes and it'd be nice to update the existing themes to add them were possible.