Define some basic cross-editor configuration

EditorConfig helps contributors make changes that fit the repo, even if
their local settings are different.

.editorconfig

@@ -0,0 +1,14 @@
+# This configures basic cross-editor formatting.
+#
+# See https://editorconfig.org/ for more info, and to see if your editor
+# requires a plugin to take advantage of it.
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 4
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true