WIP: Highlight history modifiers (#485) #697
                
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This is #485 (comment), rebased and with the tests written (for reasons explained there).
Concerns:
:t2/:h2modifier (new since Highlight parameter expansion history modifiers #485) - https://github.com/zsh-users/zsh/blob/e5765bfdc89f131d03051ba4963819fdfe252503/README#L49-L63