Create Semgrep Action

[NicholasEllul]

Summary

This pull request creates a Semgrep plug-in for the MetaMask Security Code Scanner allowing us to leverage semgrep in our scans.

Testing this action

1. Clone the https://github.com/metamask/appsec-Playground/ repository. It currently uses MetaMask/Security-Code-Scanner@ellul/add-semgrep which is a branch of the code scanner configured to use this pull requests code.
2. Open a pull request in Appsec-Playground
3. See that the scan runs semgrep successfully
4. I've already created an example pull request you can review here) which also showcases how test directories are ignored.