(core) add grist.getTable(tableId) and a getTableId() method in plugin api

Summary:
Makes the new TableOperations API available for all tables
in the document. Adds methods for discovering the tableId of the
selected table. I was very tempted to implement the select() TODO
in the TableOperations API, but it requires a significant refactor
of the backend.

Test Plan: added test

Reviewers: alexmojaki

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D3325

app/plugin/TableOperations.ts

@@ -19,6 +19,9 @@ export interface TableOperations {
   upsert(records: Types.AddOrUpdateRecord|Types.AddOrUpdateRecord[],
          options?: UpsertOptions): Promise<void>;
 
+  // Determine the tableId of the table.
+  getTableId(): Promise<string>;
+
   // TODO: offer a way to query the table.
   // select(): Records;
 }