Add highlighting configuration for Tex

[znculee]

Thanks for the terrific color scheme!

I found the original color for math symbol is too close to their surroundings, which make the equations are hard to read as follows. After changing the color for math symbol, the equations seem better as follows. In addition, the original EasyMotionTarget is bold red, but it is linked to search currently, which makes it hard to see clearly as follows. If somebody wants to use search for EasyMotionTarget, it still can be set in one's own configuration file. I think we should keep the original setting in EasyMotion by default, and this is actually easier to see as follows.

I would also prefer to define more elaborate colors for Tex, and you are welcome to give any suggestions or ideas.

[znculee]

I also change the styles of underscore, superscript, and subscript as normal math color.

[znculee]

Red statements seem uncomfortable, so I redefine the statement, refzone and etc. referring to the gruvbox used by Overleaf.