add benchmark action that leaves comment on PR (#588)

* better benchmarks

benchmark each detector with negative and positive inputs

* add benchmark action that leaves comment on PR

.github/workflows/benchmark.go

@@ -0,0 +1,40 @@
+name: Run benchmarks
+on:
+  pull_request:
+    branches: [master]
+
+permissions:
+  contents: read
+
+jobs:
+  benchmark:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v5.0.2
+      with:
+        go-version-file: 'go.mod'
+    - run: go install golang.org/x/perf/cmd/benchstat@latest
+    // Base for comparison is master branch.
+    - name: Checkout code
+      uses: actions/checkout@v4.1.7
+      with:
+        ref: master
+    - run: go test -run=none -bench=. --count=7 > /tmp/prev &
+
+    - name: Checkout code
+      uses: actions/checkout@v4.1.7
+    - run: go test -run=none -bench=. --count=7 > /tmp/curr &
+
+    // Wait for both benchmarks to complete before comparing.
+    - run: wait
+    - run: RESULT="$(benchstat /tmp/prev /tmp/curr)"
+    - uses: actions/github-script@v7
+        with:
+            script: |
+                github.rest.issues.createComment({
+                    issue_number: context.issue.number,
+                    owner: context.repo.owner,
+                    repo: context.repo.repo,
+                    body: $RESULT
+                })