Diagnostics and Diiffs inconstencies

[boydkelly]

Noticed a few issues with both diagnostics, underline, and diffs. Some groups are missing and some inconsistencies as per images below. Regarding the diffs groups I did note that in the default neovm there are base groups Added, Changed, Removed. In my personal config, i have just pointed all the variations to those 3 for consistent rendiring. There are also 3 treesitter groups missing @diff.plus/minus/delta.

As for @text.underlne, is there ia reason it is magenta? Should this just link to underline as well?

I can provide pr's for any/all of these if you wish! Cheers!

[xStormyy]

I think @text.underline being linked to underline should be okay. Also sure, send a PR if you have the time!