Split up ScreenPositionSlotControl into its own .USS file

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Views/Slots/ScreenPositionSlotControlView.cs

 
         public ScreenPositionSlotControlView(ScreenPositionMaterialSlot slot)
         {
+            AddStyleSheetPath("Styles/Controls/ScreenPositionSlotControlView");
             m_Slot = slot;
             var enumField = new EnumField(slot.screenSpaceType);
             enumField.OnValueChanged(OnValueChanged);

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Resources/Styles/Controls/ScreenPositionSlotControlView.uss

+ScreenPositionSlotControlView > EnumField {
+    margin-top: 0;
+    margin-bottom: 0;
+    margin-left: 0;
+    margin-right: 0;
+    width: 54;
+}

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Resources/Styles/Controls/ScreenPositionSlotControlView.uss.meta

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+guid: 61a89c57c8aee6c43ac9433a7a5dbf9e
+ScriptedImporter:
+  userData: 
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