LibWeb/DOM: Implement the Document object's partial attributes

Author: https://github.com/PGHales
Commit: 75626c6cd2
Pull-request: https://github.com/SerenityOS/serenity/pull/23956
Reviewed-by: https://github.com/LucasChollet
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/tcl3

Tests/LibWeb/Text/expected/DOM/Document-set-partial-attributes.txt

@@ -0,0 +1,20 @@
+before document.fgColor=
+before body attribute text=null
+after document.fgColor=red
+after body attribute text=red
+before document.linkColor=
+before body attribute link=null
+after document.linkColor=red
+after body attribute link=red
+before document.alinkColor=
+before body attribute alink=null
+after document.alinkColor=red
+after body attribute alink=red
+before document.vlinkColor=
+before body attribute vlink=null
+after document.vlinkColor=red
+after body attribute vlink=red
+before document.bgColor=
+before body attribute bgcolor=null
+after document.bgColor=red
+after body attribute bgcolor=red