Create `.editorconfig`

[Ericson2314]

I almost accidentally added a tab.

[mweinelt]

And treefmt-nix did not catch it?

[Ericson2314]

Oh it probably would have, but I like have having both. (I would make a PR, but I have to run out the door in a second.)

[mweinelt]

Yeah, I guess we can add one for good editor behaviour.

[jfly]

Not opposed, but my personal preference would be to get people's editors using nix fmt or the treefmt provided by the devshell.