[Enhancement] PapyrusXTextEditor must contribute a SelectionProvider to the EditorSite

We need to ensure the same behavior as other editors declared in Papyrus. The PapyrusXtextEditor must register its own SelectionProvider on the editor site. Since this is a text editor, the selection within the editor is a textual selection. However, this type of selection is currently not relevant for our context. Therefore, the SelectionProvider should always return the editor’s context to allow the ModelExplorer to maintain synchronization. Additionally, the SelectionProvider must always return TextDocument#getSemanticContext() under all circumstances.