LibWeb/CSS: Move rule-conversion code into its own file

This is an attempt to bring the size of Parser.cpp down. No code
changes, just moves and some explicit template instantiations now that
we're using them from a different file.

Meta/gn/secondary/Userland/Libraries/LibWeb/CSS/Parser/BUILD.gn

@@ -8,6 +8,7 @@ source_set("Parser") {
     "MediaParsing.cpp",
     "Parser.cpp",
     "ParsingContext.cpp",
+    "RuleParsing.cpp",
     "SelectorParsing.cpp",
     "Token.cpp",
     "Tokenizer.cpp",

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -80,6 +80,7 @@ set(SOURCES
     CSS/Parser/MediaParsing.cpp
     CSS/Parser/Parser.cpp
     CSS/Parser/ParsingContext.cpp
+    CSS/Parser/RuleParsing.cpp
     CSS/Parser/SelectorParsing.cpp
     CSS/Parser/Token.cpp
     CSS/Parser/Tokenizer.cpp