Support other diff output formats

[Nindaleth]

It seems only one of the diff tool output styles is supported at the moment (diff -u), others are highlighted partially or not at all. I would like for the support to be a bit more complete.

Steps to reproduce:

1. Create sample files to diff:

# cat 1
hello
howdy
hello
# cat 2
hello
hello
hello

2. Use different ways to diff the files:

# diff 1 2 > diff-default.diff
# diff -u 1 2 > diff-u.diff
# diff -c 1 2 > diff-c.diff

3. Open the diffs in editors and check:

• Zed (uses this repo):

• Sublime Text (uses a different implementation):

As all diff formats seem to follow the same "first column decides the operation" rule, I'd be happy if the most impactful characters <, > and ! were added into the highlighting. The diff format-specific headers are nice to have, but I'm mainly looking for the improvement to diff.plus, diff.minus and (if it even exists) diff.change syntax types.

[clason]

(if it even exists) diff.change syntax types.

Both Helix and Neovim have @diff.delta for changes.

[clason]

Adding the markers is trivial; the challenge is to make sure the existing rules for unified diffs don't apply falsely in the context diff (without complicating the grammar too much) -- especially the --- markers, which have a very different meaning here!

PR welcome.

[clason]

The more I think about it, the less I believe that this is actually possible with reasonable effort -- these are effectively three completely different grammars, which would have to be distinguished at the root level, each with its individual rules. The corresponding queries would also get quite messy...