feat(editorconfig): add global editorconfig

We had some files with windows line endings (see cl/30139).

They're hopefully gone now, if not, make sure editors honoring
editorconfig will convert things.

Change-Id: I3915f8cd7f4a9e98350f654d03060d7a7e98ea8e
Reviewed-on: https://cl.snix.dev/c/snix/+/30140
Reviewed-by: Paul Meyer <katexochen0@gmail.com>
Tested-by: besadii
Autosubmit: Florian Klink <flokli@flokli.de>

.editorconfig

@@ -0,0 +1,10 @@
+# EditorConfig is awesome: https://editorconfig.org
+
+# top-most EditorConfig file
+root = true
+
+# Unix-style newlines with a newline ending every file
+[*]
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true