(core) Draft version of AI assistant

Summary:
The feature is behind a flag GRIST_FORMULA_ASSISTANT (must be "true"). But can be enabled in the
developer console by invoking GRIST_FORMULA_ASSISTANT.set(true).

Keys can be overriden in the document settings page.

Test Plan: For now just a stub test that checks if this feature is disabled by default.

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3815

app/client/components/GristDoc.ts

@@ -175,6 +175,9 @@ export class GristDoc extends DisposableWithEvents {
   public externalSectionId: Computed<number|null>;
   public viewLayout: ViewLayout|null = null;
 
+  // Holder for the popped up formula editor.
+  public readonly formulaPopup = Holder.create(this);
+
   private _actionLog: ActionLog;
   private _undoStack: UndoStack;
   private _lastOwnActionGroup: ActionGroupWithCursorPos|null = null;