[css mode] Fixing a vendor prefix highlighting bug

A more detailed problem description in issue #2523
[gfm mode] Stack link tokens in overlay with base tokens.

Added `combineTokens` for overlays that can override default
`combine` argument.
[real-world uses] Add ecsspert.com
Ensure current target is scrolled into view when alt-drag selecting

Issue #2518
More closer mimic Emacs behavior in transposeChars command

Issue #2507
Remove replaced code

Issue #2507