untested... test: add unit test suite and CI workflow

I have never done this style of test before... who knows if it will work

- Add Gitea Actions workflow for PHP 8.2 testing
- Add PHPUnit configuration and bootstrap file
- Add unit tests for TalkService, WebhookController, ImageCleanup, NavigationProvider, and DebugToggle
- Add README documentation for the test suite
- Update composer.json with test and lint scripts

composer.json

@@ -6,5 +6,12 @@
         "psr-4": {
             "OCA\\Ncbotwebhooks\\": "../lib/"
         }
+    },
+    "require-dev": {
+        "phpunit/phpunit": "^10.0"
+    },
+    "scripts": {
+        "test": "phpunit --configuration phpunit.xml.dist",
+        "lint": "find lib -name '*.php' -exec php -l {} \\;"
     }
 }