Missing a locale: fa-us

**Describe the bug**
Currently, there's a 'fa' locale in the repository, but it does not translate gregorian month names to Persian (Farsi) it shows "Jalali" month names instead (which is fine) but we need a translation for gregorian month names too. I think maybe a "fa-us" locale can be a good approach here.

**Expected behavior**
A new locale for the "Farsi" language will be added which translates gregorian calendar month names.

**Information**
- Day.js Version [e.g. v1.0.0]
- OS: [e.g. iOS]
- Browser [e.g. chrome 62]
- Time zone: [e.g. GMT-07:00 DST (Pacific Daylight Time)]

Start by reading the existing `fa` locale in the repository and compare its month-name translations with the requested Gregorian names. Add the proposed `fa-us` locale following the repository's locale conventions; done means Gregorian month names are translated while the existing `fa` Jalali behavior remains unchanged.

Written by the indexing model from the issue text.