Improve highlighting in diffs

[lyxell]

It would be nice with better diff highlighting, as seen in e.g. comby https://www.youtube.com/watch?v=JMZLBB_BFNg (03:20)

This could probably be achieved with tokenization and a check between upper and lower lines, no need to involve the AST.

[lyxell]

Comby also has another nice feature when displaying diffs.

For a diff like:

-       int[] arr = {1, 2, 3, 4};
+       int[] arr;

It uses highlighting to instead display

! int[] arr = {1, 2, 3, 4};

where the bold part is red.

[lyxell]

After trying out a few different options, this is probably the way to go: