Add a .editorconfig

[multun]

I noticed some of the files had weird quirks:

• white spaces at the end of lines
• missing newline at the end of files

editorconfig is a unified config format for editors to stop fighting over tiny details like that. This configuration is pretty conservative:

• 4 spaces because that's mostly the default for rust
• spaces instead of tabs because editors still can't agree on how wide a tab is
• insert a final newline because some editors will do it anyway
• remove trailing white spaces because these serve no purpose anyway

It applies to all text files in the repository, which should bring a bit more consistency overall (these settings also don't conflict with rustfmt)

[cart]

I think I would prefer rustfmt to be the "one true way" to format code. Adding more things to the mix just complicates the system and makes it harder to reason about. What we really need is rustfmt checks to be enabled in CI so we're always clean.

[multun]

Good point, superseded by #201