Send warnings to STDERR, not STDOUT

[docwhat]

In issue #1955 the warning is being sent to STDOUT which is messing up editors by adding the warning to the top of the file.

All warnings and errors should be sent to STDERR.

[ljharb]

Good call, this should be fixed.

[ahakanbaba]

Could there be a new release including this fix ?

[ljharb]

v7.12.0 is released.