Syntax Highlighting Issue

Looks like the addition of [required fields](https://github.com/cue-lang/proposal/blob/HEAD/designs/1951-required-fields-v2.md), `!:`, is causing issues in the syntax highlighting for vim.

![image](https://github.com/jjo/vim-cue/assets/384180/474b0040-8ae1-4a5c-b2bb-d5ef13c5e691)

![image](https://github.com/jjo/vim-cue/assets/384180/976674d9-f627-49d6-81de-9736c0fb912d)
