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.

src/vm/wren_core.wren.inc

@@ -80,7 +80,7 @@ static const char* coreModuleSource =
 "    return result\n"
 "  }\n"
 "\n"
-"  join { join(\"\") }\n"
+"  join() { join(\"\") }\n"
 "\n"
 "  join(sep) {\n"
 "    var first = true\n"