Fix Field type annoations in gt4py.next

[havogt]

• Change Field[[A,B,C], DType] typing annotations to a syntactically valid form (Field[Dims[A,B,C], DType]) to avoid many mypy complaints in our code, and also as preparation for using variadic generics from PEP 646.
  • Current annotation style should be still supported to avoid breaking user codes (it should be marked as deprecated at some point).
  • For now, Dims could be just an alias for Tuple.
  • FOAST and PAST parsers need to be updated to support the new annotations.
  • Probably Dimension implementation should be changed to be actual classes to avoid mypy errors and even get some meaningful typing errors for free.