Make lexical context available during template formatting

[jfeser]

This makes template formatting more like a standard format string. The following templates should work:

feet_per_mile = 5280  # noqa: F841

@Template.define
def convert(feet: int) -> float:
    """How many miles is {feet} feet? There are {feet_per_mile} feet per mile."""
    raise NotHandled

@dataclass
class User:
    name: str

    @Template.define
    def greet(self, day: str) -> float:
        """Greet the user '{self.name}' and wish them a good {day}."""
        raise NotHandled