Add no-invalid-position-declaration stylelint rule

This rule was introduced in stylelint 16.23. We were always following
it, and it's unlikely that we miss it before opening a pull request, but
it might be useful during development.

.stylelintrc.yml

@@ -28,6 +28,7 @@ rules:
   length-zero-no-unit: true
   max-nesting-depth: 4
   media-feature-name-no-vendor-prefix: true
+  no-invalid-position-declaration: true
   property-no-unknown: true
   property-no-vendor-prefix: true
   rule-empty-line-before: