Support OTP-27.0-rc1 (triple-quoted strings and sigil string literals)

[sile]

This PR adds support for the following new language features introduced in OTP-27.0-rc1:

- Triple-Quoted Strings has been implemented as per EEP 64 to allow 
   a string to encompass a complete paragraph.
- Adjacent string literals without intervening white space is now a syntax error, 
   to avoid possible confusion with triple-quoted strings.
- Sigils on string literals (both ordinary and triple-quoted) have been implemented as per EEP 66.
  - For example, ~"Björn" or ~b"Björn" are now equivalent to <<"Björn"/utf8>>.