LibJS: Shrink Identifier's environment coordinate cache

This patch does two things:

- We now use u32 instead of size_t for the hops and index fields
  in EnvironmentCoordinate. This means we're limited to an environment
  nesting level and variable count of 4Gs respectively.

- Instead of wrapping it in an Optional, EnvironmentCoordinate now has
  a custom valid/invalid state using a magic marker value.

These two changes reduce the size of Identifier by 16 bytes. :^)

Userland/Libraries/LibJS/AST.h

@@ -1222,7 +1222,7 @@ private:
     virtual bool is_identifier() const override { return true; }
 
     FlyString m_string;
-    mutable Optional<EnvironmentCoordinate> m_cached_environment_coordinate;
+    mutable EnvironmentCoordinate m_cached_environment_coordinate;
 };
 
 class PrivateIdentifier final : public Expression {