LibWeb: Construct CSS Tokenizer and Parser with a StringView encoding

This doesn't need to be a full (Deprecated)String, so let's not force it
to be.

Userland/Libraries/LibWeb/CSS/Parser/Parser.h

@@ -59,7 +59,7 @@ private:
 
 class Parser {
 public:
-    Parser(ParsingContext const&, StringView input, DeprecatedString const& encoding = "utf-8");
+    Parser(ParsingContext const&, StringView input, StringView encoding = "utf-8"sv);
     ~Parser() = default;
 
     CSSStyleSheet* parse_as_css_stylesheet(Optional<AK::URL> location);