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/Libraries/LibJS/Token.cpp

@@ -82,7 +82,7 @@ double Token::double_value() const
         }
     }
     // This should always be a valid double
-    return value_string.to_double().release_value();
+    return value_string.to_number<double>().release_value();
 }
 
 static u32 hex2int(char x)