LibJS: Deduplicate labelled continue & break bytecode generation

Similar to the scoped continue and break, the only two differences
between these functions is the scope that is scanned for a matching
label, and the specific handling of a continue/break boundary.

Userland/Libraries/LibJS/Bytecode/Generator.h

@@ -217,6 +217,7 @@ private:
         Break,
     };
     void generate_scoped_jump(JumpType);
+    void generate_labelled_jump(JumpType, DeprecatedFlyString const& label);
 
     Generator();
     ~Generator() = default;