(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/ui2018/editableLabel.ts

@@ -62,7 +62,7 @@ const cssSizer = styled('div', `
 
 enum Status { NORMAL, EDITING, SAVING }
 
-type SaveFunc = (value: string) => Promise<void>;
+type SaveFunc = (value: string) => void|PromiseLike<void>;
 
 export interface EditableLabelOptions {
   save: SaveFunc;