format code (editorconfig and clang-format)

.editorconfig

@@ -0,0 +1,11 @@
+root                     = true
+
+[*]
+indent_style             = space
+indent_size              = 3
+tab_width                = 3
+end_of_line              = lf
+charset                  = utf-8
+trim_trailing_whitespace = true
+insert_final_newline     = true
+max_line_length          = 120