Basic unit test for workspace viewer

[ElianHugh]

• Introduces (very basic) unit testing for the workspace viewer

Also proposes unit testing through mocking the vscode API via the sinon library. E.g., we can "set" vscode.window.activeTextEditor via the following function:

export function mockActiveTextEditor(document: vscode.TextDocument, sandbox: sinon.SinonSandbox) {
    return sandbox.stub(vscode.window, 'activeTextEditor').value({
        document
    });
}

[renkun-ken]

Really nice work!