These will be fallen back to when selection.background isn't
defined.

Fixes #28554

#28197

start with a step back and use IWorkspace where needed, nuke workspace context service from ext host, #28526

This reverts commit 25b4b172.

Remove ArraySet and move to native Set

Fixes #28138

The new selection model in xterm.js doesn't allow inverting colors
right now so piggyback on selection.background. Ideally the terminal
would have its own selection key since it has a terminal.background,
but this is hopefully just temporary until selection color inverting
is done upstream.

Fixes #28389