fix: Require missing iconv and mbstring dependencies

- `ext-mbstring` is required and to allow it to work as a PHAR the
  polyfill is added.
- `symfony/polyfill-mbstring` require `iconv` (see
  symfony/symfony#52207).

composer.json

@@ -18,6 +18,8 @@
     ],
     "require": {
         "php": "^8.1",
+        "ext-iconv": "*",
+        "ext-mbstring": "*",
         "ext-phar": "*",
         "ext-sodium": "*",
         "composer-plugin-api": "^2.2",
@@ -40,6 +42,8 @@
         "symfony/console": "^6.1.7",
         "symfony/filesystem": "^6.1.5",
         "symfony/finder": "^6.1.3",
+        "symfony/polyfill-iconv": "^1.28",
+        "symfony/polyfill-mbstring": "^1.28",
         "symfony/process": "^6.1.3",
         "symfony/var-dumper": "^6.1.6",
         "webmozart/assert": "^1.11"