fix: add S and SS fractional-second tokens to format()

[JSap0914]

Bug

The base format() function only handled SSS (3-digit milliseconds). The S (hundreds digit) and SS (first two digits) tokens were absent from REGEX_FORMAT, so they were never matched — instead they appeared as literal characters in the output:

dayjs('2021-01-01T10:00:00.500').format('S')   // ❌ 'S'   (should be '5')
dayjs('2021-01-01T10:00:00.520').format('SS')  // ❌ 'SS'  (should be '52')
dayjs('2021-01-01T10:00:00.520').format('SSS') // ✅ '520'

This also broke customParseFormat strict mode for S/SS formats. Strict mode re-formats the parsed date and compares it against the original input; because format('S') returned a literal 'S', the comparison always failed:

dayjs('2021-01-01T00:00:00.52',  'YYYY-MM-DDTHH:mm:ss.SS', true).isValid() // ❌ false
dayjs('2021-01-01T00:00:00.5',   'YYYY-MM-DDTHH:mm:ss.S',  true).isValid() // ❌ false

Reported in #1331.

Fix

src/constant.js — extend REGEX_FORMAT from /SSS/ to /S{1,3}/ so that single and double S tokens are matched.

src/index.js — add two new cases to the matches() switch inside format():

Token	Returns	Example (520 ms)
S	Math.floor(ms / 100)	'5'
SS	Utils.s(Math.floor(ms / 10), 2, '0')	'52'
SSS	Utils.s(ms, 3, '0') (unchanged)	'520'

The token semantics match customParseFormat's existing parse-side expressions (S×100, SS×10, SSS×1), so strict-mode roundtrips now succeed.

Verification

npx jest test/plugin/customParseFormat_S_SS.test.js --no-coverage

PASS test/plugin/customParseFormat_S_SS.test.js
  S and SS format tokens
    base format() support
      ✓ format S returns first digit of milliseconds (hundreds)
      ✓ format SS returns first two digits of milliseconds
      ✓ format SSS still works (3 digits)
    customParseFormat strict mode with S/SS tokens
      ✓ SS strict mode parses 2-digit fractional seconds correctly
      ✓ S strict mode parses 1-digit fractional seconds correctly
      ✓ SSS strict mode still works
Tests: 6 passed

Full suite: 779 tests pass, 0 failures.

Note: this PR was developed with AI assistance.

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