Improve highlighting of format verbs in format string arguments

[antoniusnaumann]

Encountered behavior:

Formatting verbs in format arguments are highlighted the same way as the string.

Expected behavior:

Formatting verbs should be specially highlighted.

Context:

Formatting verbs such as #v or T can be appended in Goto by putting a : after the format arg, e.g.:
"I have \(appleCount:d) apples"

Resources:

Standard neovim highlighting groups:
https://neovim.io/doc/user/treesitter.html#treesitter-highlight