The zero-argument overload of `join` on `Sequence` was previously defined as a
getter-style method without parentheses. This change makes it a regular method
that requires explicit parentheses, consistent with the one-argument `join(sep)`
variant and standard Wren method syntax. The implementation now reads
`join() { join("") }` instead of `join { join("") }`.
All call sites in tests and documentation are updated accordingly.
Bob Nystrom
e0984a5012