AK+Everywhere: Rename String to DeprecatedString

We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)

Userland/Libraries/LibJS/Script.h

@@ -43,7 +43,7 @@ private:
     NonnullRefPtr<Program> m_parse_node; // [[ECMAScriptCode]]
 
     // Needed for potential lookups of modules.
-    String m_filename;
+    DeprecatedString m_filename;
     HostDefined* m_host_defined { nullptr }; // [[HostDefined]]
 };