Fix bin reference for PHAR compilation.

[kleinmann]

I've gotten errors when using the compiled PHAR and it had an inflated size due to including the already present PHAR from builds/expose. This PR fixes the PHAR configuration to match the documented output location and to use the correct main entrypoint.

[what-the-diff[bot]]

• The box.json file was changed to add an output and main property
• A new .gitignore rule was added for the expose binary in builds/expose
• composer's bin entry now points directly at the expose executable instead of a symlinked version in builds/expose

[kleinmann]

• The box.json file was changed to add an output and main property
• A new .gitignore rule was added for the expose binary in builds/expose
• composer's bin entry now points directly at the expose executable instead of a symlinked version in builds/expose

Ignore the last point, that was an oversight from earlier tests and was since removed 🙂