add phpstan to ci build

Author: dbu

Diff for: .github/workflows/.editorconfig

@@ -0,0 +1,2 @@
+[*.yml]
+indent_size = 2

Diff for: .github/workflows/static.yml

@@ -0,0 +1,21 @@
+name: Static analysis
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+
+jobs:
+  phpstan:
+    name: PHPStan
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: PHPStan
+        uses: docker://oskarstark/phpstan-ga
+        with:
+          args: analyze --no-progress