Added /UIElementsSchema to unity .gitignore

/UIElementsSchema is an auto-generated folder created by Unity. It is used in the UI Toolkit.

In Unity discussions, staff have confirmed that this folder can be safely ignored.
 
https://discussions.unity.com/t/uielementsschema/805120

Unity.gitignore

@@ -97,3 +97,6 @@ InitTestScene*.unity*
 
 # Auto-generated scenes by play mode tests
 /[Aa]ssets/[Ii]nit[Tt]est[Ss]cene*.unity*
+
+# UI Toolkit auto generated files
+/UIElementsSchema