Add a .editorconfig file for dfmt. The settings are the same as DMD's. [NFC]

Author: JohanEngelen

.editorconfig

@@ -0,0 +1,9 @@
+root = true
+
+[*.{c,h,d,di,dd}]
+end_of_line = lf
+insert_final_newline = true
+indent_style = space
+indent_size = 4
+trim_trailing_whitespace = true
+charset = utf-8