Enabling MetaMask security code scanner (#79)

This pull request enables the [MetaMask Security Code
Scanner](https://github.com/metamask/Security-Code-Scanner/) GitHub
Action. This action runs on each pull request, and will flag potential
vulnerabilities as a review comment. It will also scan this repository's
default branch, and log any findings in this repository's [Code Scanning
Alerts
Tab](https://github.com/metamask/contributor-docs/security/code-scanning).

Author: metamaskbot

.github/workflows/security-code-scanner.yml

@@ -0,0 +1,43 @@
+name: 'MetaMask Security Code Scanner'
+
+on:
+  push:
+    branches: ['main']
+  pull_request:
+    branches: ['main']
+
+jobs:
+  run-security-scan:
+    runs-on: ubuntu-latest
+    permissions:
+      actions: read
+      contents: read
+      security-events: write
+    steps:
+      - name: MetaMask Security Code Scanner
+        uses: MetaMask/Security-Code-Scanner@main
+        with:
+          repo: ${{ github.repository }}
+          paths_ignored: |
+            .storybook/
+            '**/__snapshots__/'
+            '**/*.snap'
+            '**/*.stories.js'
+            '**/*.stories.tsx'
+            '**/*.test.browser.ts*'
+            '**/*.test.js*'
+            '**/*.test.ts*'
+            '**/fixtures/'
+            '**/jest.config.js'
+            '**/jest.environment.js'
+            '**/mocks/'
+            '**/test*/'
+            docs/
+            e2e/
+            merged-packages/
+            node_modules
+            storybook/
+            test*/
+          rules_excluded: example
+          mixpanel_project_token: ${{ secrets.SECURITY_CODE_SCANNER_MIXPANEL_TOKEN }}
+          slack_webhook: ${{ secrets.APPSEC_BOT_SLACK_WEBHOOK }}