Examples: Add Lithium Carbonate to List of Examples (#622)

Fixes an oversight from #614
where the lithium carbonate example was not added to `Examples` in `__init__.py`
and was thus not selectable as a structure.

src/aiidalab_qe/app/structure/__init__.py

@@ -32,6 +32,7 @@
     ("Gallium arsenide", file_path / "examples" / "GaAs.xyz"),
     ("Gold (fcc)", file_path / "examples" / "Au.cif"),
     ("Cobalt (hcp)", file_path / "examples" / "Co.cif"),
+    ("Lithium carbonate", file_path / "examples" / "Li2CO3.cif"),
 ]
 
 OptimadeQueryWidget.title = "OPTIMADE"  # monkeypatch