Left scriptstyle

> Before requesting a feature, please search for [existing issues](https://github.com/KaTeX/KaTeX/issues).

**Is your feature request related to a problem? Please describe.**
Left scriptstyle is not inline with the right one.

**Describe the solution you'd like:**
A command that works like `a_0` but on the left side

**Link to or name of a (La)TeX package that provides the same feature:**
prescript

**Describe alternatives you've considered:**
`{}_{0}p_{0}`