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

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