This reverts commit 25b4b172.

Remove ArraySet and move to native Set

Fixes #28138

Integrate xterm.js's new terminal selection feature

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

Fixes #26038

fixes #28134

#28175