Function parseDateEntityFormat

Parse a date-time entity format string into flags for messageEntityFormattedDate.

Format string must match: r|w?[dD]?[tT]?

Control characters:

• r: relative time (cannot be combined with others)
• w: day of the week
• d: short date (e.g. "17.03.22")
• D: long date (e.g. "March 17, 2022")
• t: short time (e.g. "22:45")
• T: long time (e.g. "22:45:00")

Empty string means display text as-is, user can still see the underlying date.