[Bindings] Enable waiting for command execution

[voodoos]

Our current binding for registerCommand restricts callbacks to returning unit that prevents obtaining a promise to wait for when calling executeCommand. This could be helpful to have in the future to implement more complex workflows.

Official API:
https://vscode-api.js.org/modules/vscode.commands.html#registerCommand
https://vscode-api.js.org/modules/vscode.commands.html#executeCommand

[mnxn]

This looks related to/a duplicate of #965.