style: Add an EditorConfig file for the C code.

EditorConfig lets editors automatically configure themselves to the
project standard. Like embedded VIM and emacs mode lines, but
universal. Many editors support it out of the box. Most everything
else has a plugin. https://editorconfig.org/

It makes contributing with proper style easier. Pretty basic stuff.

* 4 spaces for indentation
* UTF-8
* Unix newlines
* Strip trailing whitespace
* Add a final newline on the file

Author: schwern

.editorconfig

@@ -0,0 +1,10 @@
+root = true
+
+[**/*.[ch]]
+charset = utf-8
+indent_style = space
+indent_size = 4
+tab_width = 8
+end_of_line = lf
+trim_trailing_whitespace = true
+insert_final_newline = true