Enhance isEmail validation

[smaye81]

This enhances the current isEmail validation by using a consistent regex that will be used across protovalidate implementations.

[timostamm]

Nice and simple. I wonder:
Is the regex compatible? See comment below.
Is this a feature branch for implementing bufbuild/protovalidate#320?

[smaye81]

Nice and simple. I wonder: Is the regex compatible? See comment below. Is this a feature branch for implementing bufbuild/protovalidate#320?

Yeah sorry, switched the main branch to use a feature branch. Meant to do this last night.

[timostamm]

Great. Can you add a docstring for is_email?

def is_email(string: celtypes.Value) -> celpy.Result:
    """Returns true if the string is an email address, for example "foo@example.com".

    Conforms to the definition for a valid email address from the HTML standard.
    Note that this standard willfully deviates from RFC 5322, which allows many
    unexpected forms of email addresses and will easily match a typographical
    error.
    """

    if not isinstance(string, celtypes.StringType):
        msg = "invalid argument, expected string"
        raise celpy.CELEvalError(msg)
    m = _email_regex.match(string) is not None
    return celtypes.BoolType(m)

See https://realpython.com/documenting-python-code/

In most other implementations, we avoid CEL types in the function, and have a small layer for the CEL bindings to converts types. I don't think we should do this in this step, but it might very well turn out to be the right thing to do.