refactor(common): use an enum as a sentinel value instead of NoMatch class

[kszucs]

This pattern plays better with type checkers, and makes it clearer that NoMatch is a singleton value rather than a singleton type (i.e. consumers should check for is NoMatch rather than isinstance(x, NoMatch)).

class NoMatchType(enum.Enum):
    NoMatch = "NoMatch"

NoMatch = NoMatchType.NoMatch

This way if we ever wanted to type annotate the match methods below, their signature could be:

def match(self, value: Value) -> Value | NoMatchType:
    ...

See https://peps.python.org/pep-0484/#support-for-singleton-types-in-unions for reference.

Originally posted by @jcrist in #6041 (comment)