keymap: Allow modifiers as keys (#12047)

It is sometimes desirable to allow modifers to serve as keys themselves
for the purposes of keybinds. For example, the popular keybind in
jetbrains IDEs `shift shift` which opens the file finder.

This change treats modifers in the keymaps as keys themselves if they
are not accompanied by a key they are modifying.

Further this change wires up they key dispatcher to treat modifer change
events as key presses which are considered for matching against
keybinds.


Release Notes:

- Fixes #6460

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Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>