(core) Use GristObjCode in CellValue

Summary: Makes type checking a bit stronger

Test Plan: it just has to compile

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3065

app/plugin/GristData.ts

@@ -1,4 +1,20 @@
-export type CellValue = number|string|boolean|null|[string, ...unknown[]];
+// Letter codes for CellValue types encoded as [code, args...] tuples.
+export const enum GristObjCode {
+  List            = 'L',
+  Dict            = 'O',
+  DateTime        = 'D',
+  Date            = 'd',
+  Skip            = 'S',
+  Censored        = 'C',
+  Reference       = 'R',
+  ReferenceList   = 'r',
+  Exception       = 'E',
+  Pending         = 'P',
+  Unmarshallable  = 'U',
+  Versions        = 'V',
+}
+
+export type CellValue = number|string|boolean|null|[GristObjCode, ...unknown[]];
 
 export interface RowRecord {
   id: number;