Add mixins-1.vim

https://www.w3.org/TR/2025/WD-css-mixins-1-20250515/

after/syntax/css/mixins-1.vim

@@ -0,0 +1,2 @@
+syn keyword cssFontProp contained result
+syn match cssFontDescriptor "@function\>" nextgroup=cssFontDescriptorBlock skipwhite skipnl

test/test.css

@@ -763,6 +763,10 @@ mask {
   mask-type: auto;
 }
 
+@function mixins-1() {
+	result: auto;
+}
+
 .motion-1 {
   offset-path: auto;
   offset-distance: auto;