AK: Move String::number entirely to header file

Use SFINAE to enforce the fact that it's supposed to only be called for
Arithmetic types, rather than counting on the linker to tell us that an
instantiation of String::number(my_arg) was not found. This also adds
String::number for floating point types as a side-effect.

AK/String.h

@@ -252,8 +252,8 @@ public:
         return vformatted(fmtstr, VariadicFormatParams { parameters... });
     }
 
-    template<typename T>
-    static String number(T);
+    template<typename T, typename EnableIf<IsArithmetic<T>::value>::Type* = nullptr>
+    static String number(T value) { return formatted("{}", value); }
 
     StringView view() const;