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/DevTools/HackStudio/Debugger/VariablesModel.cpp

@@ -112,7 +112,7 @@ static Optional<u32> string_to_variable_value(StringView string_value, Debug::De
     }
 
     if (variable.type_name == "int") {
-        auto value = string_value.to_int();
+        auto value = string_value.to_number<int>();
         if (value.has_value())
             return value.value();
         return {};