Use file picker quick pick for app location step

[alexweininger]

Show a file picker quick pick for the app location step. The current step uses an input box, and a file picker would provide more context and make it easier for users.

View comment where it was suggested to see more details https://github.com/microsoft/vscode-azurestaticwebapps/issues/465#issuecomment-892266091

Design considerations:

• What if they have multi-root?

[alexweininger]

Docker extension has an implementation of their own: https://github.com/microsoft/vscode-docker/blob/main/src/utils/quickPickFile.ts

[alexweininger]

Tracking https://github.com/microsoft/vscode/issues/135904