Add regular expression type to language

[jclark]

This is part of #130.

[jclark]

This depends to some extent on whether we do #1098 first. If we do, then they would be a special case of this. But in either case, most things work the same:

• the regular expression type (which I will refer to as RegExp) is a subtype of anydata (we need camel case: JavaScript uses RegExp; neither RegEx nor Regex seems great to me)
• not a subtype of json
• RegExp behaves similarly to a new basic type (although Support for well-known datatypes like date/time #1098 will allow other things in that basic type)
• users can refer to RegExp type using string:RegExp
• RegExp is a subtype of readonly
• RegExp does not have storage identity (so is like string in this respect)
• == equality is based on the original string from which the regexp was parsed
• === is the same as ==
• ordering is not defined (or should we define it based on string?)
• • there is a langlib module lang.regexp (could instead call it lang.re); method call syntax r.foo() when r has type RegExp calls regexp:foo(r) similarly to other basic types
• regexp:RegExp also refers to this new basic type (probably string:RegExp is defined as regexp:RegExp)
• RegExp values can be created using a regexp-constructor-expr that uses backtick syntax
  • tag is TBD: could be regexp, re or r; current preferences is for re
  • this expression is a const expression (so const R = re`x|y`; needs to work without requiring type to be specified)
  • this uses a two-phase parse, like with xml constructor
  • regexp syntax errors are detected at compile time
  • second phase is parsed against grammar given in Specify regular expression syntax and semantics #1125
  • insertions are allowed at the same point that an atom is allowed; conceptually insertions happen after parsing; performing insertions must not lead to an invalid regular expression
  • insertions are wrapped in a non-capturing group (?:re)
  • XXX handling of flags for insertions is TBD: should they apply to inserted regexp or not?
• RegExp values can also be created completely dynamically using fromString
• toString on RegExp type works analogously to how it works on xml type
• fromJsonWithType and toJson handle RegExp type analagously to xml type

[lasinicl]

• this expression is a const expression (so const R = re`x|y`; needs to work without requiring type to be specified)

Constant expressions are limitedly supported by the compiler ATM.
ballerina-platform/ballerina-lang#13944

[jclark]

Constant expressions are limitedly supported by the compiler ATM.

That's why I added this bullet point...