Support Ruby 3.1 UCRT binary

RubyInstaller has migrate from msvcrt.dll to ucrt.dll:
https://rubyinstaller.org/2021/12/31/rubyinstaller-3.1.0-1-released.html

So that symbols should be loaded from later one.
Althouth I considered introducing a new constant like `UCRT_DLL`, this
commit still using `MSVCRT_DLL` even if Ruby is run on ucrt.dll to keep
backward compatibility. I think replacing `MSVCRT_DLL` with `UCRT_DLL`
is almost meaningless, they won't used at same time.

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

appveyor.yml

@@ -14,6 +14,8 @@ test_script:
 
 environment:
   matrix:
+    - ruby_version: "31-x64"
+    - ruby_version: "31"
     - ruby_version: "30-x64"
     - ruby_version: "30"
     - ruby_version: "27-x64"

lib/windows/api.rb

@@ -8,7 +8,9 @@ module Windows
 
   # With Microsoft Visual C++ 8 and later users should use the associated
   # DLL instead of msvcrt directly, if possible.
-  if CONFIG['host_os'].split('_')[1]
+  if RUBY_PLATFORM.split('-')[-1] == "ucrt"
+    MSVCRT_DLL = 'ucrtbase'
+  elsif CONFIG['host_os'].split('_')[1]
     if CONFIG['host_os'].split('_')[1].to_i >= 80
       MSVCRT_DLL = 'msvcr' + CONFIG['host_os'].split('_')[1]
     else