Add .editorconfig (refs #59) (#60)

Adds `.editorconfig`, copied byte-for-byte from `sneak/dnswatcher` (blob `2fe0ce0`). This is the small, safe half of #59; the `.golangci.yml` half is deferred — adopting the org-standard config surfaces ~2,990 lint findings on vaultik and needs a separate lint-cleanup decision (see #59). Hence `refs #59`, not `closes`.

`make check` and `docker build .` are green (a static config file does not affect them). Left open for review (not merged).

Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #60
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

.editorconfig

@@ -0,0 +1,12 @@
+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[Makefile]
+indent_style = tab