Add an error tests for plain-CSS mixins

Author: nex3

spec/css/mixin.hrx

@@ -0,0 +1,12 @@
+<===> error/css/mixin/input.scss
+@mixin --a {}
+
+<===> error/css/mixin/error
+Error: Sass @mixin names beginning with -- are forbidden for forward-compatibility with plain CSS mixins.
+
+For details, see https://sass-lang.com/d/css-function-mixin
+  ,
+1 | @mixin --a {}
+  |        ^^^
+  '
+  input.scss 1:8  root stylesheet