[Dot Shorthands] CFE Implementation

[kallentu]

Updates to the CFE (may be updated over time):

• Parse and add listeners in body_builder
• Add internal AST representing dot shorthands
• ExpressionGenerator for dot shorthands to create those ASTs.
• Type inference changes for finding the type
• Errors and warnings

cc. @chloestefantsova @johnniwinther @stereotype441