Allow `api.fs...` to take a path as input

[ymn64]

Instead of having to prompt the user to enter a filename after invoking api.fs.create, it would be nice if we could do something like this:

require('nvim-tree.api').fs.create('/path/to/file')

(If no input is passed, then prompt to enter filename.)

And the same for api.fs.paste.

A really good use case for this would be a user-friendly action for file duplication, which is a pretty common action.

[alex-courtis]

(If no input is passed, then prompt to enter filename.)

Maintains existing behaviour.

require('nvim-tree.api').fs.create('/path/to/file')

This can apply directly to:
- create
- remove
- trash
- rename
- cut
- paste

In the event of a file missing/lacking permissions etc. existing behaviour can be kept: appropriate error message.