add Windows support

[erip]

This PR seeks to add Windows support to this repo to close #11. As a part of this, it will transition to GitHub Actions for ease of testing.

[erip]

@levyfan CI passes here. If you want to accept this, you can add GitHub Actions support for your repo and it should guarantee the three major platforms are supported.

[erip]

And, as an aside, this PR could probably be cleaned up by someone who knows Maven better than me. 😄 But this works and isn't too invasive.

[levyfan]

Thank you @erip for your great work!

One question, can we remove the file windows-jni/include/jni.h?

[erip]

I can try, but I suspect there may be issues. Specifically on Windows, jint is an alias to a native long, which causes a lot of issues. The workaround is often to use a custom (in this case, Android-based) JNI header.

[erip]

@levyfan, as I suspected this won't work: https://github.com/erip/sentencepiece-jni/runs/1668614105?check_suite_focus=true#step:4:3561

Because a jint is a long on Windows, we can't rely on the Windows JNI. The workaround with the windows-jni/ here is the same approach we took on adding Java runtime support to PyTorch on Windows, so there's some precedent. 😅 If you're OK with adding this dir, this can be merged.