Additional TeX syntax support

[thatcomputerguy0101]

This is the syntax expansion portion of #11, updated for Math.js 12. The edit to the types included in this PR is also separated out in my math-types branch.

New TeX features:

• Variable subscripts: a_b
• Custom symbols used in multiple places, such as greek letter variables (\alpha), and certain operators.
• Comparison operator support: \ne, <, \geq, etc...
• Functions with custom bases: \sqrt[3]{x}, \log_3{y}