Everywhere: Use to_number<T> instead of to_{int,uint,float,double}

In a bunch of cases, this actually ends up simplifying the code as
to_number will handle something such as:

```
Optional<I> opt;
if constexpr (IsSigned<I>)
    opt = view.to_int<I>();
else
    opt = view.to_uint<I>();
```

For us.

The main goal here however is to have a single generic number conversion
API between all of the String classes.

Userland/Utilities/pidof.cpp

@@ -71,7 +71,7 @@ ErrorOr<int> serenity_main(Main::Arguments args)
                 return true;
             }
 
-            auto number = omit_pid_value.to_uint();
+            auto number = omit_pid_value.template to_number<pid_t>();
             if (!number.has_value())
                 return false;