Use more prominent focus indicator in trees

[Tyriar]

From #93916

The different shades of backgrounds in the explorer are a little confusing:

A border could be nicer and more obvious which is the special item, it also aligned with Windows Explorer:

Related: #95767

[joaomoreno]

What does Finder do?

[joaomoreno]

Also, this will affect users of the list/tree which use it in strange ways, outline for example. This is a big one.

Also, won't even start without a consensus from @microsoft/vscode

[Tyriar]

Finder doesn't distinguish the items:

When right clicking everything gets a white outline:

Right clicking something not in the selection will get a blue outline:

[akbyrd]

Windows resets selection on right click

Windows also changes selection when you use the arrows keys

In contrast with vscode which leaves a confusing persistent selection behind

[joaomoreno]

This is now fixed