(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/TableOperationsImpl.ts

@@ -16,6 +16,10 @@ export class TableOperationsImpl implements TableOperations {
                      private _defaultOptions: OpOptions) {
   }
 
+  public getTableId() {
+    return this._platform.getTableId();
+  }
+
   public create(records: Types.NewRecord, options?: OpOptions): Promise<Types.MinimalRecord>;
   public create(records: Types.NewRecord[], options?: OpOptions): Promise<Types.MinimalRecord[]>;
   public async create(recordsOrRecord: Types.NewRecord[]|Types.NewRecord,