Migration to monorepo

[Reinmar]

We've been using multirepo for quite some time now, but as the time goes by we see that it is being less and less productive for our use case.

• Working with the code will be simpler (now we use home made mrgit tool to do that comfortably)
• Release tools are much more complicated with multirepo, more synchronization needed.
• Many times it requires additional software (e.g. TravisCI - making PRs in multiple packages to fix one issue requires you to checkout this PR related branch for every dependency, otherwise it fails the tests)

There are multiple things to be handled in this project (along with the priority):

• Merge CKEditor 5 subrepositories to the main repository (p1)
• Update CI (p2,p3?)
• Code coverage (p3)
• Adjust dev tools
  • Testing (p1)
• Online builder - we need to make sure works well (p2)

Stage 2

• Deploy changes to the CKEditor 5 repository

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[mlewand]

Added the monorepo-mock branch, that has the file layout that we should be looking for, we can use it for further testing.

[mlewand]

All right we migrated repositories:

There's still some documentation to be updated - this will be handled at the beginning on of upcoming week.

[mlewand]

Moved followups to stage II #6746.

[robclancy]

I'm so glad you changed your mind on this (#6014). I have had to search so much to find things.