Escapable linebreaks in heregexes

...as per discussion in #3214.

(Plus a pair of forgotten parentheses.)

#### Implementation Note

If the heregex has interpolations the compilation of escaped linebreaks and literal `\n` is slightly different (but semantically the same):

    /// \n\
    #{x} ///
    
compiles to:

    RegExp("\\n\n" + x);