(core) New type conversion in the backend

Summary: This is https://phab.getgrist.com/D3205 plus some changes (https://github.com/dsagal/grist/compare/type-convert...type-convert-server?expand=1) that move the conversion process to the backend. A new user action ConvertFromColumn uses `call_external` so that the data engine can delegate back to ActiveDoc. Code for creating formatters and parsers is significantly refactored so that most of the logic is in `common` and can be used in different ways.

Test Plan: The original diff adds plenty of tests.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3240

app/common/isHiddenTable.ts

@@ -0,0 +1,12 @@
+import {UIRowId} from 'app/common/UIRowId';
+import {TableData} from "./TableData";
+
+/**
+ * Return whether a table identified by the rowId of its metadata record, should normally be
+ * hidden from the user (e.g. as an option in the page-widget picker).
+ */
+export function isHiddenTable(tablesData: TableData, tableRef: UIRowId): boolean {
+  const tableId = tablesData.getValue(tableRef, 'tableId') as string|undefined;
+  return tablesData.getValue(tableRef, 'summarySourceTable') !== 0 ||
+    Boolean(tableId?.startsWith('GristHidden'));
+}