Specify indent_size in .editorconfig

Doing this purely in the hope that it makes GitHub render code blocks with 4-space-equivalent tabs by default

.editorconfig

@@ -2,6 +2,7 @@ root = true
 
 [*]
 indent_style = tab
+indent_size = 4
 charset = utf-8
 end_of_line = lf
 insert_final_newline = true