Open the current file in the default editor

[chumion]

Hi,

I think it would be handy to be able to press a key and open the file in the default editor (like in lazygit).

Expected behaviour:
In the Files tab, pressing 'o' will open the currently highlighted file in the editor set by $EDITOR

I have this working on my machine and I can open a PR, just want to make sure I am not the only person who wants it :)

[dotdash]

Sounds useful to me. I wonder if we should respect ui.editor from the jj config here to override $EDITOR maybe with yet another override at the lazyjj level.

Note: I'll try my best to keep merging things in lazyjj for now. But my main focus is on the https://github.com/blazingjj/blazingjj fork now. See #166