fmt: add .editorconfig

.editorconfig

@@ -0,0 +1,17 @@
+root = true
+
+[*]
+indent_style = space
+end_of_line = lf
+charset = utf-8
+insert_final_newline = true
+
+[*.{rs,toml}]
+indent_size = 4
+insert_final_newline = false
+
+[*.nix]
+indent_size = 2
+
+[Makefile]
+indent_size = 4