chore(devcontainer): install Intl extension in devcontainer

verification:
```
$ php -m | grep -i '^intl$'
intl
$ php -r 'var_dump(function_exists("locale_accept_from_http"));'
Xdebug: [Step Debug] Could not connect to debugging client. Tried: localhost:9003 (through xdebug.client_host/xdebug.client_port).
bool(true)
```

.devcontainer/devcontainer.json

@@ -38,7 +38,7 @@
     "features": {
         "ghcr.io/devcontainers-extra/features/mocha:2": {},
         "ghcr.io/yassinedoghri/devcontainers/php-extensions-installer:1": {
-            "extensions": "gd"
+            "extensions": "gd intl"
         }
     },
     "forwardPorts": [