(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/AceEditor.js

@@ -187,6 +187,10 @@ AceEditor.prototype._setup = function() {
   if (this.gristDoc && this.column) {
     const getSuggestions = (prefix) => {
       const section = this.gristDoc.viewModel.activeSection();
+      // If section is disposed or is pointing to an empty row, don't try to autocomplete.
+      if (!section?.getRowId()) {
+        return [];
+      }
       const tableId = section.table().tableId();
       const columnId = this.column.colId();
       const rowId = section.activeRowId();

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;