Refactor regexes and mappings class and constants.

[silvio2402]

This is part 1 of the refactoring related to b/370922064.

• Refactor the PhoneNumberRegExpsAndMappings class and its helper functions to a new regexpsandmappings.cc file.
• Move helper functions ExtnDigits and CreateExtnPattern into the PhoneNumberRegExpsAndMappings class.
• Move helper function PrefixNumberWithCountryCallingCode into the PhoneNumberUtil class.
• Refactor some common constants to constants.h.
• Add a test for getting an example number for a type that only exists for a non-geo entity.

[markusicu]

@roubert @google/libphonenumber could this PR be reviewed & merged?
I would like to modernize/simplify the character constants & escapes in a follow-up change.

[markusicu]

see the Google-internal change number 716145382