Adds two new typeable commands: show-selection-diff-popup and yank-selection-diff

[mandx]

Adds two new typeable commands: show-selection-diff-popup and yank-selection-diff

These are similar to the reset-diff-change command, except that rather wthan apply the diff base, the unsaved buffer changes (in diff syntax) are shown in a popup (with show-selection-diff-popup) or yanked to a register (using yank-selection-diff).

This uses the new hunks_intersecting_line_ranges method from #10178,
so probably that PR should be merged first before considering merging this one.

The command's names, description and aliases could be improved.

[the-mikedavis]

These seem unnecessary to me since we have :reset-diff-change. After resetting you can see the old code inline and you can yank it to the clipboard with <space>y. And then you can undo to go back to the newer code

[mandx]

The intention is to improve on convenience and ergonomics. When using :reset-diff-change to look the previous version of the hunk, it is now out context, we "lost" the new version, so to see them both at the same time, I would have to yank the old code, put it in a new buffer, go back to the prev buffer, undo, yank the new version, and paste it in the new buffer alongside the old code.

Having this in a popup have many advantages:

• It is a single command, so it's easy to map to any keybinding / chord.
• It is an actual diff, so with syntax highlighting we can see both the old code alongside the new one, and the red/green coloring clearly shows prev vs current
• It also prevents current buffer modifications, which would trigger highlighting and LSP events, which in turn can pollute the screen with warning/errors/etc because probably the old code won't compile/typecheck/lint/etc. Most times when using :reset-diff-change it becomes distracting, like, I just wanted to glance at the old hunk, ran :reset-diff-change and now I have errors and squiggly lines everywhere; one now has to find/look the old code within all that noise.