Enhancements for updating existing pull requests in GitHub integration.

[homie-gg[bot]]

. Added new script update-pull-request.ts in src/lib/github for managing PR updates.

• Modified writeCode function in src/queue/jobs/write-code.ts to utilize the new update functionality.
• Created send-pull-request-updated-message-to-slack.ts to notify Slack about updated pull requests.
• Enabled updating existing pull requests with new changes when existing_pr_number is provided in the payload.