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/LibWeb/HTML/Window.cpp

@@ -235,7 +235,7 @@ static T parse_boolean_feature(StringView value)
         return T::Yes;
 
     // 4. Let parsed be the result of parsing value as an integer.
-    auto parsed = value.to_int<i64>();
+    auto parsed = value.to_number<i64>();
 
     // 5. If parsed is an error, then set it to 0.
     if (!parsed.has_value())