LibWeb/CSS: Split up Parser.cpp

This file has been a pain to edit for a while, even with the previous
splits. So, I've divided it up into 3 parts:
- Parser.cpp has the "base" code. It's the algorithms and entry-points
  defined in the Syntax spec.
- ValueParsing.cpp contains code for parsing single values, such as a
  length, or a color, or a calculation.
- PropertyParsing.cpp contains code for parsing an entire property's
  value. A few of these sit in a grey area between being a property's
  value and a value in their own right, but the rule I've used is "is
  this useful outside of a single property and its shorthands?"

This only moves code, with as few modifications as possible to make that
work. I did add explicit instantiations for the template implementations
as part of this, which revealed a few that are actually only compatible
with a single type, so I'll clear those up in a subsequent commit.

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

@@ -8,10 +8,12 @@ source_set("Parser") {
     "MediaParsing.cpp",
     "Parser.cpp",
     "ParsingContext.cpp",
+    "PropertyParsing.cpp",
     "RuleParsing.cpp",
     "SelectorParsing.cpp",
     "Token.cpp",
     "Tokenizer.cpp",
     "Types.cpp",
+    "ValueParsing.cpp",
   ]
 }