Support `--fix-dry-run` flag

[jopemachine]

I think it would be great if xo could format the source file in vim automatically.

https://github.com/jose-elias-alvarez/null-ls.nvim/pull/707#issuecomment-1050118227

In the above PR, xo was added as a linter, but had not been added as a formatter since fix flag automatically save the changes to the file system.

It seems supporting --fix-dry-run flag might help this kind of library to support xo as formatter.

Many thanks to this awesome library.

Ref: https://eslint.org/docs/user-guide/command-line-interface#options