* Combine characters together in all expressions
Call `tryCombineChars` in `buildExpression`
* update screenshots, exclude 'coverage' from lint
* update MathML snapshots
* don't combine two 'mbin's or two 'mord's
* update screenshots
* update screenshots
Co-authored-by: ylemkimon <y@ylem.kim>
Co-authored-by: Kevin Barabash <kevinb@khanacademy.org>