fix: require parentheses for zero-argument join() method on Sequence

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.

test/core/range/join.wren

@@ -1,4 +1,4 @@
 var a = 1..3
 
-System.print(a.join) // expect: 123
+System.print(a.join()) // expect: 123
 System.print(a.join(", ")) // expect: 1, 2, 3