(core) Porting back AI formula backend

Summary: This is a backend part for the formula AI.

Test Plan: New tests

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: cyprien

Differential Revision: https://phab.getgrist.com/D3786

app/common/AssistancePrompts.ts

@@ -0,0 +1,11 @@
+import {DocAction} from 'app/common/DocActions';
+
+export interface Prompt {
+  tableId: string;
+  colId: string
+  description: string;
+}
+
+export interface Suggestion {
+  suggestedActions: DocAction[];
+}