(core) add grist.selectedTable.create/update/destroy/upsert to custom widget api

Summary: This makes an equivalent of the /records REST endpoint available within custom widgets. For simple operations, it is compatible with https://github.com/airtable/airtable.js/. About half of the diff is refactoring code from DocApi that implements /records using applyUserActions, to make that code available in the plugin api.

Test Plan: added tests

Reviewers: alexmojaki

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D3320

app/plugin/GristData.ts

@@ -16,6 +16,7 @@ export const enum GristObjCode {
 }
 
 export type CellValue = number|string|boolean|null|[GristObjCode, ...unknown[]];
+export interface BulkColValues { [colId: string]: CellValue[]; }
 
 export interface RowRecord {
   id: number;