Allow VariableReference as NamedArgument value

[eemeli]

This will need a rebase once #217 is merged. Only the last commit is really part of this PR. Edit: done.

This corresponds to the changes proposed in projectfluent/fluent#379.

[functionzz]

Mostly looks good to me but I left a few things to check over.

For fluent.runtime/fluent/runtime/resolver.py, there is a suspicious definition that may need to be changed, perhaps to:

class NamedArgument(FTL.NamedArgument, BaseResolver):
    name: Identifier
    value: Union[NumberLiteral, StringLiteral, FTL.VariableReference]

[functionzz]

Maybe also add a test that checks for no args like for test_functions.py?

    def test_missing_variable_named_arg(self, bundle):
        val, errs = bundle.format_pattern(
            bundle.get_message("thing-variable-arg").value, {}
        )
        assert val == "the thing"
        assert errs == [FluentReferenceError('Unknown external: art')]

[functionzz]

Looks good to me.