Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Libraries/LibGfx/Vector2.h

@@ -29,7 +29,7 @@ template<typename T>
 struct Formatter<Gfx::Vector2<T>> : Formatter<StringView> {
     ErrorOr<void> format(FormatBuilder& builder, Gfx::Vector2<T> const& value)
     {
-        return Formatter<StringView>::format(builder, value.to_string());
+        return Formatter<StringView>::format(builder, value.to_deprecated_string());
     }
 };