Add tests combining at rules with plain CSS nesting (#2105)

See sass/dart-sass#2322

Author: nex3

spec/css/plain/style_rule.hrx

@@ -0,0 +1,12 @@
+<===> input.scss
+@use "plain";
+
+<===> plain.css
+a {+ b {c: d}}
+
+<===> output.css
+a {
+  + b {
+    c: d;
+  }
+}