Add css-grid-2.vim

This new syntax file is for CSS Grid Layout Module Level 2.

See also: https://www.w3.org/TR/2018/WD-css-grid-2-20180206/

test/test.css

@@ -374,6 +374,10 @@
   grid-area: auto;
 }
 
+.grid-2 {
+  display: subgrid;
+}
+
 .images-3 {
   object-fit: contain;
   object-position: cover;