(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/common/gristUrls.ts

@@ -583,6 +583,9 @@ export interface GristLoadConfig {
   // TODO: remove when comments will be released.
   featureComments?: boolean;
 
+  // TODO: remove once released.
+  featureFormulaAssistant?: boolean;
+
   // Email address of the support user.
   supportEmail?: string;