Emacs can do that quite easily[1]. But this code is not merged into the trunk yet, it should be some time this year.

[1]: https://share.karthinks.com/prog-preview-3.mp4

It's not quite a preview, but back in the 90's there was something called sym-lock.el that would render TeX escape sequences as the corresponding symbol. I have no idea if it still works, but Google points me here:

  https://github.com/sid137/emacs/blob/master/sym-lock.el

(I used it with xemacs at the time, because it had more flexible font handling.)