Add composer.json with basic GitLab-CI and GitHub Actions

Add composer.json, following best practices for TYPO3 Projects.
Add GitLab-CI and GitHub Actions,
to ensure composer.json is valid and install is possible.

Relates: #5, #6

.github/workflows/ci.yml

+name: CI
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v1
+      - uses: php-actions/composer@v1
+        with:
+          command: validate
+      - uses: php-actions/composer@v1

.gitignore

+/config/
+/private/
+/public/
+/var/
+/vendor/

.gitlab-ci.yml

+stages:
+    - lint
+    - setup
+
+lint:composer:
+    image: composer:latest
+    stage: lint
+    dependencies: []
+    script:
+        - composer validate
+
+setup:composer:
+    image: composer:latest
+    stage: setup
+    dependencies: []
+    script:
+        - composer install --no-progress --no-ansi --no-interaction -o
+    artifacts:
+        expire_in: 1 hour
+        paths:
+            - vendor
+            - public
+            - private

composer.json

+{
+    "name": "typo3/example-project",
+    "description": "TYPO3 Example project, demonstrating best practices.",
+    "type": "project",
+    "license": "GPL-2.0-or-later",
+    "config": {
+        "php": "7.4",
+        "sort-packages": true
+    },
+    "require": {
+        "helhum/typo3-console": "^6.3",
+        "helhum/typo3-secure-web": "^0.2.10",
+        "typo3/cms-backend": "^10.4",
+        "typo3/cms-core": "^10.4",
+        "typo3/cms-extbase": "^10.4",
+        "typo3/cms-extensionmanager": "^10.4",
+        "typo3/cms-filelist": "^10.4",
+        "typo3/cms-fluid": "^10.4",
+        "typo3/cms-frontend": "^10.4",
+        "typo3/cms-install": "^10.4",
+        "typo3/cms-recordlist": "^10.4",
+        "typo3/cms-setup": "^10.4"
+    },
+    "extra": {
+        "typo3/cms": {
+            "root-dir": "private",
+            "web-dir": "public"
+        }
+    }
+}