feat: add run option for postponing validation until the document is saved

[aleclarson]

This adds the stylelint.run option with possible values of onType (the default) and onSave. The onType value exhibits Stylelint's current behavior. The onSave value prevents validation until the document is saved. Note that, when onSave is used, this PR doesn't explicitly clear (potentially outdated) diagnostics as you type.

Which issue, if any, is this issue related to?

#445

Is there anything in the PR that needs further explanation?

N/A

[aleclarson]

Doesn't seem to be working. I tried debugging with Launch Extension and Attach to Server launch commands, but sourcemaps seem to be broken and stepping through the compiled bundle is an awful experience thanks to private member syntax.