Problem: In addition to PgUp/PgDown Emacs also binds `Ctrl-V` to page
down and `Meta-V` to page up. These keys wouldn't extend the selection
in Zed.

Reason: Only PageUp/PageDown were assigned to
`editor::SelectPage{Up|Down}` in the `Editor && selection_mode` context.

Solution: In the `Editor && selection_mode` context, bind `Ctrl-V` to
`editor::SelectPageDown` and `Alt-V` to `editor::SelectPageUp`, both in
the mac and linux keymaps.

Release Notes:

- Added to the Emacs keymap bindings for Ctrl/Alt-V in the selection
mode to extend the selection one page up/down