Implemented Laravel 13 --langswitch compatibility.

[askdkc]

• Regex now accepts optional : void.
• Generated callback consistently includes : void.
• Test fixture updated to Laravel 13’s signature.
• Full suite passes: 12 tests, 93 assertions.
• PHPStan and Pint pass.

[what-the-diff]

PR Summary

• Updated Function Declaration in InstallLanguageSwitcher.php
  The declared function within the middleware now explicitly states it does not return any output by including a void return type. This change increases code readability and predictability, making it clearer to developers what they should expect when using this function.

• Adjusted Regular Expression in InstallLanguageSwitcher.php
  We've made a modification in the regular expression used within this file that now accurately recognizes optional return type declaration. This aspect allows for greater flexibility when declaring a function by not making it compulsory for a function to specify a return type.

• Reflected Function Syntax Changes in app.php.stub
  To ensure consistency across our codebase, we adapted the app.php.stub file to reflect the new syntax for middleware function with the void return type. This way, developers creating new applications using this stub will automatically benefit from the updated, more explicit, syntax.