Add an option to avoid wchar APIs on Windows (#3636)

With this, fmt can be used on Windows 98 and the Original Xbox with:

    set(FMT_OS OFF)

It is not exposed as a CMake option but one can define it manually
in the fmt subproject, e.g.:

    target_compile_definitions(fmt PUBLIC FMT_WINDOWS_NO_WCHAR)

Fixes #3631

test/format-impl-test.cc

@@ -351,7 +351,7 @@ TEST(format_impl_test, write_dragon_even) {
   if (!FMT_MSC_VERSION) EXPECT_EQ(s, "33554450");
 }
 
-#ifdef _WIN32
+#if defined(_WIN32) && !defined(FMT_WINDOWS_NO_WCHAR)
 #  include <windows.h>
 
 TEST(format_impl_test, write_console_signature) {