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/SVG/ViewBox.cpp

@@ -30,7 +30,7 @@ Optional<ViewBox> try_parse_view_box(StringView string)
     while (!lexer.is_eof()) {
         lexer.consume_while([](auto ch) { return is_ascii_space(ch); });
         auto token = lexer.consume_until([](auto ch) { return is_ascii_space(ch) && ch != ','; });
-        auto maybe_number = token.to_float();
+        auto maybe_number = token.to_number<float>();
         if (!maybe_number.has_value())
             return {};
         switch (state) {