When :w non-existing directory, suggest :w! to create it

[matklad]

If I do hx does-not-exist-yet/file.rs and then :w, I get the error message about directory not existing. It would help if the error message mentioned :w! as a way to create a directory.

Perhaps we can just change

helix/helix-view/src/document.rs

Line 575 in bdeefbf

bail!("can't save file, parent directory does not exist");

to be

"can't save file, parent directory does not exist (use :w! to create it)"

I am somewhat hesitant to do just that, as that is a somewhat low-level function and it's not necssary called in an interactive context.

I think the proper way to do this is to wrap some magical type like ErrorWithInterractiveHint into anyhow, and then downcast it at the call-site, but that seems like a lot of machinery which I am reluctant to add.