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/bt.cpp

@@ -33,7 +33,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     }
 
     while (iterator.has_next()) {
-        pid_t tid = iterator.next_path().to_int().value();
+        pid_t tid = iterator.next_path().to_number<pid_t>().value();
         outln("thread: {}", tid);
         outln("frames:");
         auto symbols = Symbolication::symbolicate_thread(pid, tid);