Add a way to open existing file

[Andarist]

I had to create such workaround for this: https://github.com/atlassian/changesets/blob/337b282859fb913fcafb338c563f8b928b284fd8/packages/cli/src/commands/add/index.ts#L83-L95

My use case here was not to read what has been put into the external editor - I've just wanted to open an editor, nothing else really.

I could work on implementing this here - please first let me know if that's something you'd like to support though.