(core) Allow assistant to evaluate current formula

Summary:
Replaces https://phab.getgrist.com/D3940, particularly to avoid doing potentially unwanted things automatically.

Adds optional fields `evaluateCurrentFormula?: boolean; rowId?: number` to `FormulaAssistanceContext` (part of `AssistanceRequest`). When `evaluateCurrentFormula` is `true`, calls a new function `evaluate_formula` in the sandbox which computes the existing formula in the column (regardless of anything the AI may have suggested) and uses that to generate an additional system message which is added before the user's message. In theory this could be used in an interface where users ask why a formula doesn't work, including possibly a formula suggested by the AI. For now, it's only used in `runCompletion_impl.ts` for experimenting.

Also cleaned up a bit, removing `_chatMode` which is always `true` now, and uses of `regenerate` which is always `false`.

Test Plan: Updated `runCompletion_impl` to optionally use the new feature, in which case it now scores 51/68 instead of 49/68.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3970

app/common/AssistancePrompts.ts

@@ -27,6 +27,8 @@ export interface FormulaAssistanceContext {
   type: 'formula';
   tableId: string;
   colId: string;
+  evaluateCurrentFormula?: boolean;
+  rowId?: number;
 }
 
 export type AssistanceContext = FormulaAssistanceContext;
@@ -39,10 +41,8 @@ export interface AssistanceRequest {
   context: AssistanceContext;
   state?: AssistanceState;
   text: string;
-  regenerate?: boolean;  // Set if there was a previous request
-                         // and response that should be omitted
-                         // from history, or (if available) an
-                         // alternative response generated.
+  // TODO this is no longer used and should be removed
+  regenerate?: boolean;
 }
 
 /**