Add support for PHAR executables

[TravisCarden]

Due to an issue with Symfony Process, Composer Stager does not currently detect Composer if it has a .phar extension. If that issue isn't resolved, we could work around it by searching for executable, and if it's not found, trying again with executable.phar. It would be a little ugly, but it would work if we decide it's important.

• [x] See if the issue can be fixed upstream in https://github.com/symfony/symfony/pull/52679.
• [ ] Figure out if it needs special handling, e.g., passing it as an argument to php instead of executing it directly.
• [ ] Remove it from Limitations and known issues.

11/19/2024 - As discovered in https://github.com/php-tuf/composer-stager/pull/399, it will take a little more than just finding a PHAR to run it properly--at least on Windows.

[TravisCarden]

FYI @tedbow @phenaproxima: This could affect Automatic Updates if we find a lot of hosts have Composer installed with a .phar extension.