Prompt user to save before running commands (e.g. formatting)

A few times people have asked about running commands like import sorting, formatting, etc. on modified (not saved) or brand new files (i.e. Untitled-1). We should notify users that the command does not work on such files so they don’t get a weird error message talking about e.g. EACCES.

[brettcannon]

And when Don says “do what others do”, that means researching what other extensions do (or ask VS Code on their thoughts on the topic).

[adamzev]

I’d much rather that isort just works on unsaved files than a prompt to save if that’s possible (at least for files that aren’t brand new). My workflow is often, add a new import to a file in a random spot, run sort imports and then save. I guess my workflow for the tool to work correctly needs to be, add an import, save, sort, save which adds an extra save step. I’m still experiencing .isort.cfg being ignored so I’m assuming that #4433 was closed without a fix pending this issue.