Add support for PHP 8+

[jmas]

Hello! I'm interesting in your project for using it in my pet project for edit and run Lua code as JSFiddle.

I found that antlr/antlr4-php-runtime version is outdated. It cause error during installation:

% composer require raudius/luar
./composer.json has been updated
Running composer update raudius/luar
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - antlr/antlr4-php-runtime 0.5.1 requires php 7.4 - 8.0 -> your php version (8.1.27) does not satisfy that requirement.
    - raudius/luar[0.1.0, ..., 0.1.4] require antlr/antlr4-php-runtime ^0.5.1 -> satisfiable by antlr/antlr4-php-runtime[0.5.1].
    - Root composer.json requires raudius/luar * -> satisfiable by raudius/luar[0.1.0, ..., 0.1.4].

You can also try re-running composer require with an explicit version constraint, e.g. "composer require raudius/luar:*" to figure out if any version is installable, or "composer require raudius/luar:^2.1" if you know which you need.

I'm using PHP version:

% php -v
PHP 8.1.27 (cli) (built: Dec 19 2023 20:35:55) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.1.27, Copyright (c) Zend Technologies
    with Zend OPcache v8.1.27, Copyright (c), by Zend Technologies

If I install Luar with ignoring platform requirements it works well. I checked https://github.com/antlr/antlr-php-runtime repo and found that antlr have newer version that supports PHP 8+. Probably it will be easy to upgrade antlr/antlr-php-runtime to latest.

Thanks!