(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/gutil.ts

@@ -0,0 +1,9 @@
+import constant = require('lodash/constant');
+import times = require('lodash/times');
+
+/**
+ * Returns a new array of length count, filled with the given value.
+ */
+export function arrayRepeat<T>(count: number, value: T): T[] {
+  return times(count, constant(value));
+}