HereDoc string, "+=" for dynarray and "in" string

- HereDoc string is now the prefered way to write multiline-strings -
using double-quote (`"hello world"`).

- Single quoted string  can now only be one-liners, like in the rest of
the pascal-world

- Better error-handling when strings are missing ending quotes, and
related stuff.

- The IN operator can now be used to check if a string exists in another
string (`'hello' in 'hello world'`)

- "+=" can now be used to append items to a dynamic array.