Send Next.js Integration test results to Slack (#3591)
This uses Slack's GitHub action [0] to send the contents of the summary
markdown file to a custom workflow for posting in Slack.
Since the markdown file includes newlines and quotes, this uses `jq` to
escape it and format a new json file that the action reads.
Test Plan: Manually run the workflow against this branch
[0] https://github.com/slackapi/slack-github-action
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Co-authored-by: OJ Kwon <1210596+kwonoj@users.noreply.github.com>
