closes #15205

In This PR, instead of listing the shortcuts to be shown in each category of the help modal, I'm update the code to automatically show the registered shortcuts of that category.

The good news is that this allows third-party plugins to register their shortcuts and they'll show up magically on that help modal. The only requirement is to choose one of these categories (block, selection, global).

Down the road, we can probably open to random categories but this is a good start I think.

The downside of this PR is that we don't control the order of the shortcuts. We could add an alphabetical order or a priority property per shortcut. But we can iterate on that.

Testing instructions

• Register a custom shortcut like so (paste into the console):

wp.data.dispatch('core/keyboard-shortcuts').registerShortcut( {
			name: 'mynamespace/myshortcut',
			category: 'global',
			description: 'Awesome shortcut that sends spaceships to another Galaxy',
			keyCombination: {
				character: 's',
			},
		} );

• Open the keyboard shortcuts modal, you should see the shortcut added to the right category.