Expose Tabular Data Viewer extension api for other extensions to load datasets in our Table View via standard vscode commands api

[RandomFractals]

so that other extension authors can use table view for their data display.

Example:

const rows = await db.query(statement);
vscode.commands.executeCommand(`tabular.data.viewTable`, rows);

This tabular data viewer api should povide a simple load/add data and filter methods for other 3rd party extensions that have query panels or get their data in other ways ...

Most likely how we'll add rapidsai cuDF data integration for their large data toolkit described in #6.

See example in vscode docs for setting up public api exposed by an extension via its activation method:

https://code.visualstudio.com/api/references/vscode-api#extensions