`:highlight Group font=xyz` doesn't do anything.

Hi! I wanted to try to do this trick in MacVim, basically using two different fonts at the same time (specifically Fira Code for operator ligatures and Operator Mono for everything else).

Looking through the docs, it seemed like setting the font on a highlight group was my best bet (like :highlight MyGroup font='Fira Code:h16') but it didn't work. If i misspell the font's name it generates an error which it doesn't normally, so it's clearly recognizing the font-name. But nothing happens, it doesn't actually apply the font to the highlight group in MacVim (I also checked that the group was a valid group by changing guifg=white, and it is valid). I'm using Core Text renderer, but it seems like it doesn't work either way.

I don't know if this is a problem with core vim or MacVim, but it seemed to be a GUI issue, so I'm reporting it here. If it's a known limitation of MacVim, it should probably be noted in the docs.

BTW, given that this is my first time communicating with this project: thanks for all your hard work and for providing the best GUI version of Vim there is! MacVim is so much better than the options on other OS's, at least as far as I've tried.

Start by reproducing the :highlight font behavior in MacVim with and without the Core Text renderer, using the documented highlight-font setting and a valid highlight group. Trace whether the font attribute is handled by MacVim or core Vim, then confirm whether the font is applied or the limitation should be documented.