Adding phpstan and psalm

.github/workflows/static.yml

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+on: [ pull_request ]
+name: Static analysis
+
+jobs:
+  phpstan:
+    name: PHPStan
+    runs-on: ubuntu-20.04
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: 8.1
+          extensions: apcu, redis
+          coverage: none
+          tools: phpstan:1.4.6, cs2pr
+
+      - name: Download dependencies
+        uses: ramsey/composer-install@v1
+
+      - name: PHPStan
+        run: phpstan analyze --no-progress --error-format=checkstyle | cs2pr
+
+  psalm:
+    name: Psalm
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: 8.1
+          extensions: apcu, redis
+          coverage: none
+          tools: vimeo/psalm:4.22.0
+
+      - name: Download dependencies
+        uses: ramsey/composer-install@v1
+
+      - name: Psalm
+        run: psalm --no-progress --output-format=github

phpstan-baseline.neon

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+parameters:
+	ignoreErrors:

phpstan.neon.dist

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+includes:
+  - ./phpstan-baseline.neon
+
+parameters:
+  level: 5
+  reportUnmatchedIgnoredErrors: false
+  paths:
+    - src

psalm.baseline.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<files psalm-version="4.21.0@d8bec4c7aaee111a532daec32fb09de5687053d1"/>

psalm.xml

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+<?xml version="1.0"?>
+<psalm
+    errorLevel="5"
+    resolveFromConfigFile="true"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xmlns="https://getpsalm.org/schema/config"
+    xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
+    errorBaseline="psalm.baseline.xml"
+>
+    <projectFiles>
+        <directory name="src" />
+        <ignoreFiles>
+            <directory name="vendor" />
+        </ignoreFiles>
+    </projectFiles>
+</psalm>