Add Semgrep to the MetaMask Security Code Scanner

[EllusionN]

Summary

This pull request integrates https://github.com/MetaMask/semgrep-action into the MetaMask Security Code scanner, allowing us to be capable of more easily writing custom rules.

Testing

I've set up https://github.com/MetaMask/Appsec-Playground to use this branch of the code scanner. To test this action perform the following:

1. Create a pull request in the appsec-playground repo with any conents.
2. See the scan executes successfully.

Here is an example of a rule violation: https://github.com/MetaMask/Appsec-Playground

[witmicko]

👍