Allow files to be moved to non-existing directories

[plneple]

Adds handling for non-existing directories for the MOVE command, similar to how CREATE works

[plneple]

@stevearc I was unaware that this idea has already been implemented, my bad.

There are several potential solutions I was able to come up with:

• ~~Merge as-is. If you replace the last step in a scenario you described with a simle mv command, the outcome is exactly the same, with no warnings or additional flags.~~

  mv a.txt foo/a.txt

• Raise an error when moving a file to an existing directory. This covers how I personally use this feature.
• Introduce a warning before / or while performing the action, if any file happens to be overwritten. I assume this would require adding not-yet existing functionality to the plugin.

Please let me know if you find any of these acceptable.

EDIT: After reading the linked PR more carefully, I now see that the first option is not acceptable for this plugin.

[stevearc]

The ideal would be that if you :w with these changes, the save operation is blocked, an error is displayed, and a diagnostic is added to the offending line. This is currently how it works for similar behavior (e.g. creating two files with the same name).

Alternatively, there is a subset of this functionality that may be easier to add: allowing these files to be moved into a new directory, but not an existing one. This doesn't produce the same issues with filename collisions, and might be easier to code.