fix(rules/en): parse 12 am as midnight

[Shaisolaris]

The English Hour and HourMinute rules treat 12 am as noon:

w := when.New(nil)
w.Add(en.All...)
r, _ := w.Parse("at 12:30am", time.Date(2026, 1, 31, 10, 30, 0, 0, time.UTC))
// r.Time is 2026-01-31 12:30, expected 00:30

The same happens for 12am, 12 A.M. and 12:00 am tomorrow. The pm branch already leaves 12 as is and adds 12 to the other hours; the am branch used the hour as written, so 12 stayed 12. This maps 12 am to hour 0 in both rules.

Testing:

• New fixtures in hour_test.go (at 12 am, 12A.M.) and hour_minute_test.go (12:30am, at 12:00 am, plus 12:30pm to pin the pm side) fail before the change and pass after.
• go test ./... and go vet ./... pass.

The br and ru hour rules have the same am branch; I left them out because I can't confirm how 12 is written with those markers in those languages.

[olebedev]

Reviewed the midnight conversion in both English time rules. Tests and go vet pass on the PR combined with current master (Go 1.26.7).