AK: Add operator""_{short_,}string to create a String from a literal

We briefly discussed this when adding the new String type but couldn't
settle on a name. However, having to use String::from_utf8() on every
literal string is a bit unwieldy, so let's have these options available!

Naming-wise '_string' is not as short as 'sv' but should be relatively
clear; it also matches '_bigint' and '_ubigint' in length.
'_short_string' may be longer than the actual string itself, but it's
still an improvement over the static function :^)

Since our C++ source files are UTF-8 encoded anyway, it should be
impossible to create a string literal with invalid UTF-8, so including
that in the name is not as important as in the function that can receive
arbitrary data.

AK/FlyString.h

@@ -69,6 +69,11 @@ struct Formatter<FlyString> : Formatter<StringView> {
 
 }
 
+[[nodiscard]] ALWAYS_INLINE AK::ErrorOr<AK::FlyString> operator""_fly_string(char const* cstring, size_t length)
+{
+    return AK::FlyString::from_utf8(AK::StringView(cstring, length));
+}
+
 #if USING_AK_GLOBALLY
 using AK::FlyString;
 #endif