Implement preview on highlighting quickpick in quick search

[andreamah]

It would be nice to show a preview editor that changes in the background based on which quick search item you have open. Furthermore, it'd be nice to highlight the result like we do in the symbol editor.

This might be sort of non-trivial to tackle, as existing pickers that change the background editor only deal with a single open editor. In our case, we would need to implement a preview editor for this.

[bpasero]

@andreamah I think this is a duplicate of #8939

I think one challenge here is that while you show preview editors in the background, you do not accumulate or alter the editor state (there is some discussion around this in the referenced issue). For example, I would expect:

• that cancelling quick pick restores editor UI as it was
• that the editors that open in the background do not appear in the editor history (e.g. "Go Back")

We currently do not really have support for the latter.

That said, I find it nice how ST shows preview editors in the background without even opening a tab. Notice how the editor opens without a tab associated to it:

//cc @TylerLeonhardt