[Experiment] Enum annotation sugar

[Kingwl]

Seems not a big feature(2-4 lines for core), but a nice sugar.

Fixes #16464

[Kingwl]

@typescript-bot pack this.

[typescript-bot]

Heya @Kingwl, I've started to run the tarball bundle task on this PR at aadd9cd. You can monitor the build here.

[typescript-bot]

Hey @Kingwl, I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/87482/artifacts?artifactName=tgz&fileId=589E795A3F50B70EDD434977F6DDF87141191CE66212826349422EDEEC02744E02&fileName=/typescript-4.1.0-insiders.20201015.tgz"
    }
}

and then running npm install.

---

There is also a playground for this build.

[typescript-bot]

The TypeScript team hasn't accepted the linked issue #16464. If you can get it accepted, this PR will have a better chance of being reviewed.

[Kingwl]

Thanks!

[wangziling]

Thanks @Kingwl, very helpful~

[sandersn]

This experiment is pretty old, so I'm going to close it to reduce the number of open PRs.