[2.3] Add support for completion when on Symfony 5.4

[dkarlovi]

According to this tweet:
https://twitter.com/seldaek/status/1461670175976169478

Composer 2.3+ will be on Symfony 5.4 which got support for completion in Console. See Symfony 5.4 adding completion to their own commands.

The proposal here is to add this feature directly into Composer, allowing custom completion where appropriate, for example

1. composer require sa<TAB> could search Packagist for packages with name sa%
2. composer update <TAB> could complete package names already in composer.json

etc.

The completion script could then be generated and shipped when Composer is distributed as a native package (for example, I'm using a package prepared by Remi), meaning many users will get DX improvement by default, others can run composer completion bash > ~/.bash_completion.d/composer.sh (or similar) once to get the same features.

[GromNaN]

I gave a try in #10320. Covering more commands than what was identified in this issue.

[dkarlovi]

Added a note about this being available in 2.4 on Remi repo issue tracker (they do RPMs for CentOS, RHEL, Fedora, etc)
remicollet/remirepo#211