feat: add support for vendor-dir

[rajyan]

• Added or updated tests
• Documented user facing changes
• Updated CHANGELOG.md

Changes
Changed $rootDir to $vendorDir and use self::$composer['config']['vendor-dir'] if exists in ApplicationResolver.php.
This PR adds support for https://getcomposer.org/doc/06-config.md#vendor-dir.

Without this PR I encountered an error, because we have our app files in laravel/app and the vendor dir is laravel/vendor.
Our Laravel project has a bootstrap file, but couldn't read it for the same reason. (I know it's a rare case, but I believe this PR helps other people using "vendor-dir")

ErrorException thrown in /var/source/app/laravel/vendor/nunomaduro/larastan/src/ApplicationResolver.php on line 134 while loading bootstrap file /var/source/app/laravel/vendor/nunomaduro/larastan/bootstrap.php: include(/var/source/app/vendor/composer/autoload_psr4.php): failed to open stream: No such file or directory

Breaking changes

This won't break any existing features, because it would fallback to the general behaviour if the self::$composer['config']['vendor-dir'] doesn't exist.

[canvural]

Thank you!