Add rule for missing backslash in Greek symbols.

[dengesCU]

Is your feature request related to a problem? Please describe.
I sometimes forget the backslash in formulas. For example, $beta$ instead of $\beta$. This compiles without an issue, but of course looks not as intended.

Describe the solution you'd like
Add a rule that gives a warning for Greek letters without a preceding "" could be added.

PS: I hope I wrote this in the right place, and thank you for the great work. If there's anything I can help with, let me know.