Clean-up parsing of timezone strings

[s0600204]

Notably:

• De-duplicate the repeated instances where a string containing a timezone name is parsed into something usable.
• To facilitate this, a new method (timezoneStringToDateTimeZone) that converts from any recognised timezone id string into a php DateTimeZone object has been added.
• The isValidCldrTimeZoneId method now only checks for validity (like the isValidIanaTimeZoneId method).
• Parse Windows timezones without having to set an option:
  • An isValidWindowsTimeZoneId method has been added, checking against the list of known Windows timezones.
  • The isValidTimeZoneId method has been adapted to use the above.

[u01jmg3]

Great work - appreciate the clean-up