Use the new "Fix All in Workspace" commands in the "dart fix" prompt

[DanTup]

When opening a project that has diagnostics that can be fixed with "dart fix" we currently direct you to instructions to run it from the command line:

When using a pre-release SDK that supports the new "Fix All in Workspace" commands, you instead get an option to trigger those commands and see the changes in-editor:

The reason this is currently only enabled for pre-release SDKs is that the UX for the Preview option isn't great - all of the entries are unticked by default (and it can be tedious to tick them). I have an open PR for VS Code I hope may improve this at microsoft/vscode#210175. If that (or something similar) doesn't land soon, we may wish to consider enabling this functionality anyway (or perhaps hiding the Preview button?).