Investigate multistep quickpicks

[ewanharris]

The current quickpick flow for building/creation/packaging etc. kinda strings the various quickpicks together, meaning it can be cancelled accidentally at any time (this can be overridden by setting isCancellable to false iirc). It seems to be possible to build a MultiStepInput that creates a better UX for theses flows.

References,

https://github.com/Microsoft/vscode-extension-samples/blob/master/quickinput-sample/src/multiStepInput.ts
microsoft/vscode#49340