[widgets][experimenteditor] Create experiment asset with generated content

This content is not valid in the sense that it's mostly empty.

beat/editor/widgets/experimenteditor.py

@@ -1373,6 +1373,8 @@ class ExperimentEditor(AbstractAssetEditor):
                 AssetType.EXPERIMENT,
                 os.path.join(user, toolchain_name, experiment_name),
             )
+            os.makedirs(os.path.dirname(asset.declaration_path), exist_ok=True)
+            asset.declaration = experiment_declaration
             return asset, experiment_declaration
 
         else: