Editorial: Add two more required formats

[anba]

This makes format selection in BasicFormatMatcher unambiguous when requesting the following two formats:

1. Standalone "month":

There's currently a tie between "year, month" and "month, day". Adding standalone "month" as a required format resolves this tie.

2. "dayPeriod, hour, minute, second, fractionalSecondDigits":

There's a tie between "hour, minute, second, fractionalSecondDigits" and "dayPeriod, hour, minute, second". Requiring a format which has both "dayPeriod" and "fractionalSecondDigits" resolves this tie.

[sffc]

LGTM but is it editorial?

[anba]

If we consider these ties as spec bugs, I'd say this is editorial. Otherwise it's normative, because it adds new requirements for implementations. Except no (non-experimental) implementation actually implements BasicFormatMatcher, so implementers can likely just ignore this PR.

(Even if there's no actual implementation for BasicFormatMatcher, it's still useful to keep it in the spec, because it defines the minimum requirements for BestFitFormatMatcher.)