HereDoc string, "+=" for dynarray, "In" support for strings.

A few changes..:
- HereDoc / multiline strings now requires <"> (double quote).
- Single quoted strings are now single line only.
- Better error handling when string lacks ending quotation-mark.
- The `+=` operator supports dynamic arrays (appending item).
- The `In` operator can now be used to test if a string contains a sub-string.