LibJS: Add modification counter in DeclarativeEnvironment

This counter is incremented whenever a mutating operation occurs
within the environment's set of bindings.

It is going to be used by GetGlobal instruction to correctly invalidate
cache when global declarative environment is mutated.

Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.cpp

@@ -77,6 +77,8 @@ ThrowCompletionOr<void> DeclarativeEnvironment::create_mutable_binding(VM&, Depr
         .initialized = false,
     });
 
+    ++m_environment_serial_number;
+
     // 3. Return unused.
     return {};
 }
@@ -97,6 +99,8 @@ ThrowCompletionOr<void> DeclarativeEnvironment::create_immutable_binding(VM&, De
         .initialized = false,
     });
 
+    ++m_environment_serial_number;
+
     // 3. Return unused.
     return {};
 }
@@ -217,6 +221,8 @@ ThrowCompletionOr<bool> DeclarativeEnvironment::delete_binding(VM&, DeprecatedFl
     // NOTE: We keep the entries in m_bindings to avoid disturbing indices.
     binding_and_index->binding() = {};
 
+    ++m_environment_serial_number;
+
     // 4. Return true.
     return true;
 }