Add .editorconfig

Consistent code style across editors.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>

.editorconfig

+# EditorConfig is a file format and collection of text editor plugins
+# for maintaining consistent coding styles between different editors
+# and IDEs. Most popular editors support this either natively or via
+# plugin.
+#
+# Check https://editorconfig.org for details.
+
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+
+[*.c]
+indent_style = space
+indent_size = 4
+
+[*.{rng,xml}]
+indent_style = space
+indent_size = 2