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.

• See if the issue can be fixed upstream in [Process] ExecutableFinder::addSuffix() has no effect symfony/symfony#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 #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.