LibWeb: Make resolved serialization of CSS display prefer short form

Although we translate e.g `block` to `block flow` for internal use in
the engine, CSS-DISPLAY-3 tells us to use the short form in
serializations for compatibility reasons.

This adds 9 points to our score on https://html5test.com/ :^)

Tests/LibWeb/Text/expected/css/getComputedStyle-display-short-serialization.txt

@@ -0,0 +1,14 @@
+none => none
+block => block
+flow-root => flow-root
+inline => inline
+inline-block => inline-block
+run-in => run-in
+list-item => list-item
+flex => flex
+inline-flex => inline-flex
+grid => grid
+inline-grid => inline-grid
+ruby => ruby
+table => table
+inline-table => inline-table