Add proposal for support C++20 Modules

[PikachuHyA]

No description provided.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[lberki]

Mind linking this into README.md under the "under review" section?

[lberki]

Thanks! It appears that you also need to sign our CLA (I missed this yesterday, sorry about that)

My understanding is that you work for Alibaba and my data shows that Alibaba Group already has a corporate CLA signed, which means that you should be covered, but our bot seems to disagree. Instructions are here to fix it; you probably either need to add the e-mail address to the set of people covered by your CLA or use a different e-mail on your git commit.

Sorry about the red tape.

[PikachuHyA]

It appears that you also need to sign our CLA

done

[lberki]

Thanks! I'll merge this now.