.editorconfig: insert a newline at end-of-file

This keeps `git` from "complaining" about this when doing a diff.  It
also keeps your prompt at the left edge when you `cat` a file.

.editorconfig

@@ -9,4 +9,5 @@
 root = true                     ; look no further
 
 [*]
+insert_final_newline = true
 trim_trailing_whitespace = true