cartesian: parser gets tripped by `Field | None`

[romanc]

Description

Gtscript stencils (and functions) can have optional arguments, e.g. as mentioned in

NOAA-GFDL/NDSL#524 (comment)

However, if we write the same in "new style" syntax, i.e. Field | None instead of Optional[Field], the parser trips and we crash.

This issue is about adding support for the "new style" syntax because we should just support both.

[romanc]

According to NOAA-GFDL/NDSL#557, this also happens for union types, i.e. Float | Int instead of Union[Float, Int]

[FlorianDeconinck]

This is not true - cartesian stencils cannot have optional fields for now. The comment in NDSL#524 refers to the orchestrated __call__ method and not the stencils.

Trying to have an optional field on the stencil like so

    input_field: FloatField,
    output_field: FloatField,
    optional_output_field: Optional[FloatField] = None
):
    """All stencil code should live in the global space."""

    with computation(PARALLEL), interval(...):
        output_field = input_field
        optional_output_field = input_field

gives

gt4py.cartesian.frontend.exceptions.GTScriptValueError: Invalid annotated dtype value for argument 'optional_output_field': typing.Optional[_FieldDescriptor(dtype=dtype('float64'), axes=(Axis(name=I), Axis(name=J), Axis(name=K)), data_dims=())]