The major change with the unit tests was no longer requiring
a secondary null field for dispatchparts. As chords can be any length,
it's more forward-thinking to modify the unit tests to accept new behavior.

It was difficult to refactor the ResolvedKeybindings without
also refactoring label creators, as the null checks where handled in the
label generation.

As a start to support key chords of longer than two parts,
modifying the internal structure of ChordKeybinding to not specifically
call out two parts.

Use `Array<T | U>` when dealing with union types instead of  `(T | U)[]`

We compile using the alwaysStrict flag so these directives are not needed

fixes #57789

Fixes #28237: Identify keyboard layouts where A-Z are missing or unusual and assume the standard mapping for KeyA - KeyZ

- Implement APIs in ResolvedKeybinding to expose parts
- Implement APIs in ResolvedKeybinding to get labels without modifiers
- Adopt tests to above API changes
- Use above APIs to search and highlight keybindings
- A new keybinding label widget to render a keybinding
- Adopt above widget in Keyboard shortcuts editor and define keybinding widget