Add Psalm to GitHub Actions

.github/workflows/static-analysis.yml

@@ -0,0 +1,31 @@
+---
+name: Static analysis
+
+on:
+  pull_request:
+  push:
+    paths:
+      - '**.php'
+      - '*psalm.xml'
+
+jobs:
+  psalm:
+    name: Psalm
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v1
+        with:
+          php-version: 7.4
+          coverage: none
+
+      - name: Cache Composer dependencies
+        uses: actions/cache@v1
+        with:
+          path: vendor
+          key: composer-${{ hashFiles('composer.lock' )}}
+
+      - name: Run Psalm
+        run: make static-code-analysis