Improve local dev setup with Composer and PHPUnit in Docker container

- Remove dependency on globally installed Composer
- Provide a way to install and run Composer and PHPUnit in the container created by wp-env
- Start basic PHPUnit tests in addition to existing unit tests (using wp-now) and E2E tests (Playwright)
- Document local dev/test/build setup

Author: eliot-akira

.gitignore

@@ -8,7 +8,6 @@ node_modules
 *.local.js
 
 # Ignore lock files to allow any package manager
-composer.lock
 package-lock.json
 yarn.lock
 bun.lockb

composer.json

@@ -1,3 +1,10 @@
 {
-  "name": "tangible/framework"
+  "name": "tangible/framework",
+  "repositories": [],
+  "require-dev": {
+    "phpunit/phpunit": "^9.6",
+    "yoast/phpunit-polyfills": "^2.0"
+  },
+  "require": {},
+  "minimum-stability": "dev"
 }