add .editorconfig for consistent code style across editors (#22916)

EditorConfig is a well-known convention to share style settings across
different editors. Adding one will make it easier for new contributors
or people who like to use a different editor to contribute.

.editorconfig

@@ -0,0 +1,6 @@
+root = true
+
+[*.{c,cpp,cc,h,hpp}]
+indent_style = tab
+tab_width = 8
+max_line_length = 120