Set up Semgrep scan

Add a CI Job to run a Semgrep scan on this codebase. The job is
configured to run only on the `main` branch because it requires access
to the `SEMGREP_APP_TOKEN` secret which may not be accessible on PRs
from external contributors. The job will upload the report as a SARIF
file to GitHub for ingestion without having to rely on the Semgrep web
application.

Signed-off-by: Eric Cornelissen <[email protected]>

.github/workflows/semgrep.yml

@@ -0,0 +1,47 @@
+# MIT No Attribution
+#
+# Copyright 2024 Eric Cornelissen
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy of this
+# software and associated documentation files (the "Software"), to deal in the Software
+# without restriction, including without limitation the rights to use, copy, modify,
+# merge, publish, distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+# INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+# PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+# HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+# SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+name: Semgrep
+on:
+  push:
+    branches:
+      - main
+
+permissions: read-all
+
+jobs:
+  semgrep:
+    name: Semgrep
+    runs-on: ubuntu-24.04
+    permissions:
+      security-events: write # To upload SARIF results
+    container:
+      image: semgrep/semgrep
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          persist-credentials: false
+      - name: Perform Semgrep analysis
+        run: semgrep ci --sarif --output semgrep.sarif
+        env:
+          SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}
+      - name: Upload Semgrep report to GitHub
+        uses: github/codeql-action/upload-sarif@ea9e4e37992a54ee68a9622e985e60c8e8f12d9f # v3.27.4
+        if: ${{ failure() || success() }}
+        with:
+          sarif_file: semgrep.sarif