Allow multiple keyboard shortcuts per command

[raineorshine]

Current Behavior

Commands define aliases with hideFromHelp: true to allow additional keyboard shortcuts.

For example:

em/src/commands/cursorDown.tsx

Lines 134 to 139 in 177c2a7

	export const cursorDownAlias: Command = {
	...cursorDownCommand,
	id: 'cursorDownAlias',
	gesture: undefined,
	keyboard: { key: Key.ArrowDown, shift: true },
	}

Expected Behavior

• Expand Command.keyboard to allow Key[].
• Show the first Key in the array in Command Palette, Help modal, etc.
• The commandKeyIndex will need to be adapted to support multiple keyboard shortcuts but still allow O(1) lookup.
  • Consider adding one entry per keyboard shortcut (instead of one entry per command) so that hashKeyDown can still be used.
• Remove command aliases that are no longer needed.