start an editorconfig (#1119)
start an editorconfig (#1119)

I adapted this from [nushell's
editorconfig](https://github.com/nushell/nushell/blob/main/.editorconfig)

Baby steps for #1110.
exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust