LibWeb: Initialize static web strings during main-thread VM creation

These are currently initialized in a [[gnu::constructor]], which has a
weird initialization order. These constructors are invoked before main()
and, incidentally, before any user-defined default constructors of the
static strings they are initializing.

This will become an issue when these strings are ported to FlyString,
which has a user-defined default constructor. In that scenario, when the
FlyString constructor is executed after the [[gnu::constructor]], the
strings will be "reset" to the empty string.

Instead of relying on a non-standard compiler extension here, let's just
initialize these strings explicitly during main-thread VM creation, as
this now happens in WebContent's main().

Userland/Libraries/LibWeb/Namespace.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/DeprecatedFlyString.h>
+#include <AK/Error.h>
 
 namespace Web::Namespace {
 
@@ -22,4 +23,6 @@ namespace Web::Namespace {
 ENUMERATE_NAMESPACES
 #undef __ENUMERATE_NAMESPACE
 
+ErrorOr<void> initialize_strings();
+
 }