fix(localizedFormat): fall back to English formats for lowercase tokens

[shoutoutuoadi325]

Summary

format('l') (and ll, lll, llll) throws TypeError: Cannot read properties of undefined (reading 'replace') when the current locale has no formats object, for example a custom locale registered via dayjs.locale(name, { name, weekdays, months }) (allowed by Partial<ILocale>). Parsing has the same hole: dayjs('2/5/2018', 'l', locale) swallows the error and returns Invalid Date.

Uppercase L / LL / ... already falls back to englishFormats, so the lowercase variants should too. The last fallback becomes t(formats[B] || englishFormats[B]), and formats gets a {} default since customParseFormat passes locale.formats, which can be undefined.

Locales that do define formats keep deriving lowercase tokens from their own uppercase formats. Adding lowercase keys to englishFormats directly would shadow those with the US formats (es l would become 3/5/2024 instead of 5/3/2024).

This covers the cases still reproducing in #2281: the en repro there was fixed by the extend-time patch, but any locale without a formats object still crashes.

Test plan

• locale without formats: l / ll / lll / llll format the same as moment (new test in localizedFormat.test.js)
• parsing L, l and l LT in such a locale returns the parsed date instead of Invalid Date (new test in customParseFormat.test.js)
• npm test (all TZ variants, 100% line coverage threshold) and npm run lint pass

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