Implement """triple-quoted""" multi-line strings

[Rangi42]

Fixes #589

[Rangi42]

@ISSOtm I know you said you preferred heredoc-like syntax, but this fit neatly into the current lexer without having to add a user-defined token.

[Rangi42]

If this won't be accepted, I can try implementing custom-identifier heredoc-style instead. (Although one thing I like about triple-quotes is that they don't have to be multi-line; for example:

SENTENCE EQUS """One "fish" two "fish" red "fish" blue "fish"."""

vs.

SENTENCE EQUS <<<SEUSS ; arbitrary identifier
One "fish" two "fish" red "fish" blue "fish".
SEUSS ; languages aren't consistent on whether the trailing newline is included (Ruby) or not (Racket)

[aaaaaa123456789]

As I see it, one does not preclude the other. Both forms of syntax are so unobtrusive with respect to "normal" coding that they can easily coexist.
As for whether the trailing newline should be included or not in a heredoc-like string, I think not including it would be the best idea, simply because it's easy to add an extra newline if needed, but impossible to remove it if not.

[ISSOtm]

Above and beyond with the new test cases! 👍