Generate checker functions for data type rules

[spoenemann]

It's easy to generate checker functions for rules that consist of an alternative of string literals. We might even consider extending this to terminal rules by using their corresponding regular expression, but it might be hard to check arbitrary data type rules (which can be composed of multiple lexer tokens).

Originally posted by @goto40 in #389 (comment)

Is there a function generated by the langium code generator to check the validity of a string concerning a given union type from the grammar? E.g., isState in the example above.