Warn against overwriting directories

[mandx]

Warn against overwriting directories if the destination path doesn't end with a directory separator.

• Create a directory test-dir and a test-file.txt inside (so we end with test-dir/test-file.txt)
• Open Helix in the current directory
• Use :n to create a new buffer and add some content to it
• Use :w test-dir to save the new buffer (note the directory name is used without the trailing slash)
• Observe that now the buffer was correctly saved to test-dir as a file
• However the directory itself was not lost, but renamed with a temporary/backup name (something like test-dirp4F0MM.bck/).

While not a critical data loss bug (the backup-rename of the original directory happens every time) it would be nice to prevent this situation in the first place, very much like we warn against external modifications by checking modification time.

[kirawi]

It would be good to have a test for this in https://github.com/helix-editor/helix/blob/master/helix-term/tests/test/commands/write.rs

[mandx]

It would be good to have a test for this in master/helix-term/tests/test/commands/write.rs

@kirawi I just added one; I copied the test_overwrite_protection test and tweaked just enough for what we want to test.

However I didn't find a way to check for that the warning message is displayed; I see that app.editor has a status_msg field of type Option<(Cow<'static, str>, Severity)> but in the test I always get None...