We list special notices in diffs for the following conditions:

Files without any changes made
Newly-added files that are empty
Files that were deleted
Files containing whitespace-only changes
Files containing suspicious Unicode characters

These notices now all use the rb-c-diff-file-notice component, showing

an icon and defaulting to a yellow warning background on light mode and

a header color on dark mode. This gives us a color scheme that both

stands out and safely shows diff colors.
The one notice type that isn't covered is for binary files. This is kept

more plain for now, just due to differences between standard file and

defaut states for binary files.
As part of this, we've moved away from embedded rb-c-alert components.

We aren't using ink-c-alert here, but rather introducing a new

rb-c-diff-file-notice, which is specific to the diff viewer. This lets

us ensure a structure that works well in this context, with the visuals

we need, and without having to override styles from either rb-c-alert

or ink-c-alert to make it fit correctly.

Unit tests pass.
Tested all notice combinations, making sure the visuals looked right in

light and dark modes.