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/HTMLTableElement.cpp

@@ -46,7 +46,7 @@ void HTMLTableElement::visit_edges(Cell::Visitor& visitor)
 
 static unsigned parse_border(ByteString const& value)
 {
-    return value.to_uint().value_or(0);
+    return value.to_number<unsigned>().value_or(0);
 }
 
 void HTMLTableElement::apply_presentational_hints(CSS::StyleProperties& style) const