LibWeb: Move Declaration and DeclarationOrAtRule into Parser namespace

Author: https://github.com/AtkinsSJ
Commit: add6babef4
Pull-request: https://github.com/SerenityOS/serenity/pull/13643

Userland/Libraries/LibWeb/CSS/Parser/Declaration.cpp

@@ -8,7 +8,7 @@
 #include <LibWeb/CSS/Parser/Declaration.h>
 #include <LibWeb/CSS/Serialize.h>
 
-namespace Web::CSS {
+namespace Web::CSS::Parser {
 
 Declaration::Declaration() = default;
 Declaration::~Declaration() = default;