Add Semgrep workflow for code scanning

[testtest126]

This workflow scans code using Semgrep and uploads SARIF results.

Summary

Test plan

• swift test in ChessKit/ passes
• swift test in chess-server/ passes (if server code changed)
• App builds / relevant UI tests pass (if app code changed)

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.