Add .editorconfig

This should ensure that all editors which support
https://editorconfig.org/ use the correct indentation
with spaces (not tabs) by default.

This is hardly a full specification of our coding style,
but it's a correct subset and better than nothing.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/353>

.editorconfig

@@ -0,0 +1,17 @@
+# SPDX-FileCopyrightText: 2024 Florian Müllner <fmuellner@gnome.org>
+# SPDX-License-Identifier: CC0-1.0
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+
+indent_style = space
+trim_trailing_whitespace = true
+
+[*.js]
+indent_size = 4
+
+[meson.build]
+indent_size = 2