(core) Adds setSelectedRows to the grist api for custom view

Summary:
This is needed to let custom widget driver filtering of other widget in the same page.

Descripion here:
 - https://grist.quip.com/ctytAQJoFMsM/Hopefully-Small-Projects#temp:C:NNCfe2030b27647439886ca83595

Test Plan: New api tested in a new nbrowser test

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3253

app/plugin/GristAPI.ts

@@ -99,4 +99,10 @@ export interface GristView {
 
   // Similar TODO to fetchSelectedTable for return type.
   fetchSelectedRecord(rowId: number): Promise<any>;
+
+  // Allow custom widget to be listed as a possible source for linking with SELECT BY.
+  allowSelectBy(): Promise<void>;
+
+  // Set the list of selected rows to be used against any linked widget. Requires `allowSelectBy()`.
+  setSelectedRows(rowIds: number[]): Promise<void>;
 }