LibWeb: Extract some CORS and MIME Fetch helpers to their own files

An upcoming commit will migrate the contents of Headers.h/cpp to LibHTTP
for use outside of LibWeb. These CORS and MIME helpers depend on other
LibWeb facilities, however, so they cannot be moved.

Libraries/LibWeb/HTML/Parser/HTMLEncodingDetection.h

@@ -11,6 +11,7 @@
 #include <AK/Optional.h>
 #include <LibGC/Ptr.h>
 #include <LibWeb/Forward.h>
+#include <LibWeb/MimeSniff/MimeType.h>
 
 namespace Web::HTML {