LibJS: Add basic monomorphic caching for PutById property access

This patch makes it possible for JS::Object::internal_set() to populate
a CacheablePropertyMetadata, and uses this to implement a basic
monomorphic cache for the most common form of property write access.

Userland/Libraries/LibJS/Bytecode/Generator.h

@@ -236,6 +236,7 @@ public:
 
     [[nodiscard]] size_t next_global_variable_cache() { return m_next_global_variable_cache++; }
     [[nodiscard]] size_t next_environment_variable_cache() { return m_next_environment_variable_cache++; }
+    [[nodiscard]] size_t next_property_lookup_cache() { return m_next_property_lookup_cache++; }
 
 private:
     enum class JumpType {