Changes

feat(scripts/pingcap): add deno script `add-placeholder-version-in-readme.ts` (#2889)

Usage:
```bash
deno run --allow-all scripts/pingcap/add-version-placeholder-readme.ts --owner <owner> --github_private_token <token> --repo <repo> --branch master --placeholder 8.2.0-alpha --draft
```
Signed-off-by: wuhuizuo <wuhuizuo@126.com>

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Signed-off-by: wuhuizuo <wuhuizuo@126.com>
 (commit: 86e9b54)