Show warning / error when input/output are the same

[rudiv]

This is one to help stupid people (like me, who did exactly this earlier today).

When you totally accidentally run:

tailwindcss -i site.css -o site.css

Show a message that you are being stupid, perhaps with nicer wording, so that the user has at least a chance to not lose their entire CSS file that may or may not be version controlled at the time.

[adamwathan]

Ah yep great idea, definitely something we can do 👍

[rudiv]

Just realised how simple it would be, so submitted a PR. Can't find any tests from a brief look (I didn't look very hard) so it's just the edit to the cli index.ts.

#17311