Create retext plugin: Dates

[danielgolden]

Requirements

When a user breaks one of these rules, provide feedback (See one core docs):

• Write dates with either the full month or a 3 letter month followed by the day number.
• Include the 4-digit year wherever possible to clarify dates for international markets where we don't localize. If the year is already obvious, leave it off.
• Don’t use ordinal indicators like 1st, 2nd, 3rd, 4th, etc.
• Don't use number-only dates like 12/5/2019, as these don't mean the same thing to many international users.
• Use an en dash (option + -) with a single space on each side when displaying date ranges.