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.

builtin/core.wren

@@ -78,7 +78,7 @@ class Sequence {
     return result
   }
 
-  join { join("") }
+  join() { join("") }
 
   join(sep) {
     var first = true