fix: parse fractional seconds shorter than 3 digits as a decimal fraction

[lenamonj]

REGEX_PARSE captures fractional seconds as bare digits and parseDate
truncates them with substring(0, 3) without right-padding, so a fraction
shorter than three digits is read as an integer count of milliseconds:

dayjs('2019-03-25T06:41:00.5').millisecond()  // 5  - moment and new Date() give 500
dayjs('2019-03-25T06:41:00.05').millisecond() // 5  - moment and new Date() give 50

ISO 8601 fractional seconds are a decimal fraction. Strings with a trailing
Z already take the native Date path and are unaffected; only the regex
path is hit.

Fix: right-pad the capture to three digits before truncating. Fractions of
three or more digits are unchanged (.999999999 still parses as 999), and
the no-fraction path still yields 0.

Test: one new case in test/parse.test.js beside the existing
parses unlimited millisecond test, comparing valueOf() and
millisecond() against moment for 1-, 2-, and 3-digit fractions. Without
the fix it fails (1 failed, 26 passed in the file); with it the file is 27
passed and the full suite is 774 passed across 93 suites, line coverage
unchanged at 100%.