Create a new D9 platform

Problem/Motivation

We had a meeting in DrupalDevDays with Gabor and Neil about porting the localize platform to D9.
As part of this meeting we decided to create a separate issue to handle the future D9 platform code.

Remaining tasks

• Create a D9 platform
• Embed l10n_server module with composer.
• Comment dependency on the l10n_pconfig module to remove the blocking issue on our migration path (for now).
• Share it with the volunteers
• Ultimately, ask for maintainership rights if needed at some point.