Add .editorconfig (see editorconfig.org) (#2943)

This file uses the same settings which we currently have in our vim modeline,
but can be picked up by many different editors without having to add and
maintain editor-specific modelines in all of our source files.

.editorconfig

@@ -0,0 +1,11 @@
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+
+[*.{c,h}]
+indent_style = space
+indent_size = 4
+trim_trailing_whitespace = true