Add emacs keybindings for mark emulation (#22904)

These keybindings extend the already selected text. This allows closer
emacs emulation where subsequent movement commands extend / shrink the
current selection instead of dismissing it.

This is a follow up on 
- #21927

Release Notes:

- Added emacs movement keybindings that extend/shrink the current
selection

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Co-authored-by: Peter Tripp <peter@zed.dev>