Restructure workflows, tests and dev packages (#86)

- refactors GitHub's actions to run a matrix of tests for PHP >= 7.0
- implements
[PHPUnit-Polyfills](https://github.com/Yoast/PHPUnit-Polyfills)
- reorganizes test files for ease of access and extension
- simplifies composers, exposing one slick file for library users
- separates concerns, grouping dev-related files in the `dev` directory

### What problem does it fix?
- tests are now run for all required PHP versions
- there were 2 top-level composer files, now only the right one is
exposed

### How to test if it works?
- create a branch and set up a PR, the CI should run and test your code
against all PHP & OS versions
- expect at least 27 checks

.github/workflows/phpunit-tests-run.yml

@@ -0,0 +1,47 @@
+name: Run PHPUnit tests
+
+on:
+  workflow_call:
+    inputs:
+      os:
+        description: 'Operating system to run tests on'
+        required: false
+        type: 'string'
+        default: 'ubuntu-latest'
+      php:
+        description: 'The version of PHP to use, in the format of X.Y'
+        required: true
+        type: 'string'
+      phpunit-config:
+        description: 'The PHPUnit configuration file to use'
+        required: false
+        type: 'string'
+        default: 'phpunit.xml.dist'
+env:
+  LOCAL_PHP: ${{ inputs.php }}-fpm
+  PHPUNIT_CONFIG: ${{ inputs.phpunit-config }}
+
+jobs:
+  phpunit-tests:
+    name: ${{ inputs.os }}
+    runs-on: ${{ inputs.os }}
+    timeout-minutes: 20
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '${{ inputs.php }}'
+          tools: phpunit-polyfills
+
+      - name: Install Composer dependencies
+        uses: ramsey/composer-install@v3
+        with:
+          ignore-cache: "yes"
+          composer-options: "--optimize-autoloader"
+
+      - name: Run PHPUnit tests
+        run: phpunit tests --testdox

.github/workflows/phpunit-tests.yml

@@ -0,0 +1,25 @@
+name: PHPUnit Tests
+
+on:
+  push:
+    branches:
+      - trunk
+  pull_request:
+
+jobs:
+  test:
+    name: PHP ${{ matrix.php }}
+    uses: ./.github/workflows/phpunit-tests-run.yml
+    permissions:
+      contents: read
+    secrets: inherit
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ ubuntu-latest, macos-latest, windows-latest ]
+        php: [ '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3' ]
+
+    with:
+      os: ${{ matrix.os }}
+      php: ${{ matrix.php }}
+      phpunit-config: ${{ 'phpunit.xml.dist' }}