Add configurable Pane border _hover_ color

[zadjii-msft]

This month on "Stealing ideas from the VsCode release notes":

We don't yet support resizing panes with the mouse (that's #992). We also don't support configuring the pane border color (that's #3061 / #3327).

BUT WHEN WE DO, it would be cool to have a configurable color for the hover on the pane borders. So you would know when you've got a pane border hovered, and that it's actionable.

Also neat would be if it had that Fluent "glow"? though I suppose "reveal focus" is "xbox only", so maybe not that.

[DHowett]

Can it just be accent, and not configurable? The cursor and the "thick bar" would indicate it well enough.

[zadjii-msft]

Well I mean, I'd assume it'd be accent colored by default. We'd get to changing the color in a follow-up. I'm just tracking customizing basically every in-app color in #3327.

I could imagine someone wanting like, 1px gray borders by default, and then 2px accent border when hovered

[mdtauk]

Is the AccentColor an option as a pickable colour for any UI or console element?

[zadjii-msft]

It will be, see #5772