Add support to label rules of mixed types

Currently it’s allowed to label rules of mixed types, however it compiles to an invalid code.

example: value=integer | value=string;

The generated parser will contain a value property which type is StringContext (because it was declared later). IMHO, the most appropriate behaviour is to declare the property as ParserRuleContext (all rules) or ParseTree (rules and tokens).

[sharwell]

For whoever fixes it, make sure there is a unit test showing the following works fine:

example
  : value=integer # integerExample
  | value=string # stringExample
  ;

[KvanTTT]

We also can not use a += operator for rules with different types (table updated):

rule5
    : t5 += a+ b t5 += c+
    ;