(core) mark engine setting as experimental

Summary:
This marks the engine setting in document settings as experimental,
with a skull and cross-bones.

It also makes sure the setting is shown if PYTHON_VERSION_ON_CREATION
is set (this relates to a separate change to set the default python
version to 3).

Test Plan: manual

Reviewers: alexmojaki, dsagal

Reviewed By: alexmojaki, dsagal

Subscribers: anaisconce

Differential Revision: https://phab.getgrist.com/D3120

app/client/ui/DocumentSettings.ts

@@ -52,7 +52,7 @@ export async function showDocSettingsModal(docInfo: DocInfoRec, docPageModel: Do
             {defaultCurrencyLabel: `Local currency (${getCurrency(l)})`})
         )),
         canChangeEngine ? [
-          cssDataRow('Engine:'),
+          cssDataRow('Engine (experimental ☠ change at own risk):'),
           select(engineObs, getSupportedEngineChoices()),
         ] : null,
       ],