Allow commands to be previewed in the command palette

Use cases:

Switching schemes - I want to be able to see what the scheme will look like, before I commit to it
Changing opacity - I'm thinking we have a nested "Change opacity..."/["opacity: 0%", "opacity: 5%", "opacity: 10%"...] commands, and as they key through them, it updates in real time
Probably useful with the tab switcher

These would need to be commands that could be undo-able. Commands where we could build a "reset" command given the current state of the Terminal, and hitting esc to dismiss the palette would revert to the original state.

I'd imagine that not all commands could be previewed. Can't preview "open a new tab".

If you went from one previewable command to another of the same action type, then we don't need to reset in between. But going from a "see theme to foo"->"set opacity to 5%" commands, we'd need to undo the first, then cache the state for the second command.

Could probably have some ActionArgs implement IPreviewable, with a ActionAndArgs GenerateUndoCommand(AppLogic); method. Anything that's "preview":true would need to implement that interface.

</showerthought>

Considered use cases:

Switching to another color scheme
changing the font size
changing the font face
changing the opacity of acrylic
Switching tabs (like in the ATS, Advanced Tab Switcher #6732)

I wonder if it's as "easy" as having the "active TerminalSettings context" and manipulating that, and then either reverting to the existing one or committing the new one as the active one.