Run PHPStan as Github Action (#2)

* Depend on PHPUnit for development

In order to run the unit tests, PHPUnit is a hard dev
dependency, so I've included it in this commit, and now
I can run the unit tests as part of this PR.

* Depend on PHPStan for development

This is for willdurand#89 - to ensure correct type hints are provided
to developers who use IDEs.

* Fix object model of AcceptHeader interface

Fixes willdurand#89 - IDEs and PHPStan are happy with this implementation

* Correct return type

* Correct nonexistent Priority class to AcceptHeader

* Improve typehint - allow looser type to be returned

* Improve typehint - more accurate types as parameters

* Improve typehint - more accurate generics as parameters

* Expose script property - was only ever written

* Properly typehint associative array

* Typehint nullable string

* Match typehints of parent method

* Add PHPStan to CI

* Configure PHPUnit versions for different PHP runtimes

* Use real phpunit

.github/workflows/ci.yaml

@@ -31,4 +31,30 @@ jobs:
         uses: "ramsey/composer-install@v1"
 
       - name: "Run PHPUnit"
-        run: "vendor/bin/simple-phpunit --coverage-text"
+        run: "vendor/bin/phpunit --coverage-text"
+
+  phpstan:
+    runs-on: "ubuntu-20.04"
+
+    strategy:
+      fail-fast: false
+      matrix:
+        php-version:
+          - "7.4"
+          - "8.0"
+          - "8.1"
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: "Install PHP ${{ matrix.php-version }}"
+        uses: "shivammathur/setup-php@v2"
+        with:
+          php-version: "${{ matrix.php-version }}"
+          coverage: "pcov"
+
+      - name: "Install dependencies with Composer"
+        uses: "ramsey/composer-install@v1"
+
+      - name: "Run PHPStan"
+        run: "vendor/bin/phpstan analyse src --level 5"