Fix C++17 implementation of wxString::ToCULong()

Do use std::from_chars<unsigned long>() in it as otherwise values
greater than LONG_MAX failed to parse.

Handle negative numbers explicitly to still parse them in this function
as well, as needs to be done for compatibility.

Add a test case for the previously failing numbers.

Closes #23957.

tests/strings/strings.cpp

@@ -607,6 +607,17 @@ static const struct ToLongData
     { wxT("0x11"), 17, Number_Ok,       0 },
     { wxT("0x11"),  0, Number_Invalid,  8 },
     { wxT("0x11"), 17, Number_Ok,      16 },
+
+    {
+#if SIZEOF_LONG == 4
+      wxT("0xffffffff"),
+#elif SIZEOF_LONG == 8
+      wxT("0xffffffffffffffff"),
+#else
+    #error "Unknown sizeof(long)"
+#endif
+      (TestValue_t)ULONG_MAX, Number_Unsigned, 0
+    },
 };
 
 wxGCC_WARNING_RESTORE(missing-field-initializers)