proposal: markdownSummary

[peaceiris]

Checklist

• I am using the latest version of this action.
• I have read the latest README and followed the instructions.
• I have read the latest GitHub Actions official documentation and learned the basic spec and concepts.

Describe your proposal

markdownSummary with a URL to a comment.

Describe the solution you'd like

https://github.com/actions/toolkit/blob/main/packages/core/src/summary.ts

Describe alternatives you've considered

No response

Additional context

No response

[actions-label-commenter]

Hi, there.

Thank you @peaceiris for suggesting this.

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