(core) add a python3 button

Summary: This adds a dropdown to the document settings model in staging/dev to set the python engine to Python2 or Python3. The setting is saved in `_grist_DocInfo.documentSettings.engine`.

Test Plan: tested manually for now - separate diff needed to add runsc to jenkins setup and make this testable

Reviewers: dsagal, alexmojaki

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D3014

app/common/DocumentSettings.ts

@@ -1,4 +1,11 @@
 export interface DocumentSettings {
   locale: string;
   currency?: string;
+  engine?: EngineCode;
 }
+
+/**
+ * The back-end will for now support at most two engines, a pynbox-backed python2 and
+ * a gvisor-backed python3.
+ */
+export type EngineCode = 'python2' | 'python3';