Support "composer create project" in addition to phar distribution

[pwtyler]

Eases pain of PHP/Robo/Terminus compatibility

[stevector]

@pwtyler can you explain more why we should add another installation mechanism? From the Docs perspective, having more installation methods creates confusion and the need to explain.

[namespacebrian]

We stopped supporting composer as an installation method because users install it in the same codebase where they have other composer packages, causing composer conflicts, and contact support saying they can't install terminus, and then we have to troubleshoot their composer setup to get terminus to install correctly.

Packaging in a phar ensures a complete and atomic set of composer packages that cannot conflict with other composer packages the user may have in their environment.

[pwtyler]

The rational from @greg-1-anderson on this I believe was to provide an avenue (through "or" options in composer.json) that allowed for installing Terminus 4+ in an environment that could not run PHP 8.1 (i.e. when using Build Tools with a site running 7.4).