Remove duplicates to Vim 8.1 default runtime

This commit also changes file names and test code for maintainability.

after/syntax/css/regions-1.vim

@@ -0,0 +1,6 @@
+" TODO: create cssRegionsProp group and cssRegionsAttr group
+syn match cssFontProp contained "\<flow-\(into\|from\)\>"
+syn match cssFontProp contained "\<region-fragment\>"
+syn keyword cssFontAttr contained element content break
+syn match cssFontAttr contained "\<\(avoid-\)\=region\>"
+syn keyword cssPseudoClassId contained region