[CI] Update Slack.yml and add newcomer-alert workflow

[cpepper96]

Current Behavior
Currently the slack.yml is outdated. Also there is no automated way of notifying the slack channel when a "good first issue" has been created.

Desired Behavior
Update slack.yml and add newcomer-alert.yml to the Github workflow.

---

Resources
Add the updated slack.yml file and the newcomer-alert.yml file to the .github/workflow folder.

Files needed:
https://github.com/meshery/meshery-istio/blob/master/.github/workflows/slack.yml
https://github.com/layer5io/layer5/blob/master/.github/workflows/newcomers-alert.yml

[welcome]

Thanks for opening this issue. A contributor will be by to give feedback soon. In the meantime, please review the Layer5 Community Welcome Guide and sure to join the community Slack.

[abhijeetgauravm]

Can I work on this issue?