Add "Prompt" option to "Run Pub Get on Pubspec Changes"

[rlipscombe]

When I'm doing an interactive rebase (on Windows, if that makes a difference), and VS Code is open, changes to pubspec.yaml (as the commits are replayed) cause the VS Code extension to re-run pub get. This occasionally causes git to fail the rebase, because pubspec.lock has changed while it's attempting to replay the changes.

Ideally, the extension would hold off running pub get until the rebase had finished, but I have no idea how you'd do that.

As a compromise, it would be useful if the "Run Pub Get on Pubspec Changes" was changed from a boolean to a tri-state: "Always | Prompt | Never".

[DanTup]

I think detecting an in-progress rebase (and then when it finishes) might be tricky (and/or unreliable), but I think having an option to always prompt is reasonable (we already have a prompt for when you open a project and we detect the files are out of sync).