• 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>