Support opening a folder with goto_file

[cbr9]

When the selection points to a folder and the user uses goto_file (gf), the editor will raise an error.

I would propose that it opens a file picker with the elements of the folder, similar to when you launch helix by passing it a folder name.

Thoughts?

(I could work on a PR, with some guidance)

[woojiq]

Sounds logical for me. Though I didn't get into situations where I might need it. Probably I just don't write a lot of raw text.
If you want to start this PR, you need to start looking for the code here:

helix/helix-term/src/commands.rs

Lines 1146 to 1147 in 929eb0c

	fn goto_file(cx: &mut Context) {
	goto_file_impl(cx, Action::Replace);

And here the error occurs:

helix/helix-term/src/commands.rs

Lines 1193 to 1195 in 929eb0c

	if let Err(e) = cx.editor.open(&PathBuf::from(p), action) {
	cx.editor.set_error(format!("Open file failed: {:?}", e));
	}

To find out how to "display a picker" you can look at impl of commands like "Open file picker".