Add multi version IG build

[oijauregui]

To support building FHIR R4 or R5 versions of the IG from the same set of source .fsh files.
Including:

• Changes to _genonce script
• Introducing a _preprocessFsh script
• Files directories modification (IG-base / R4 / R5 ).
• Usage documentation in readme.md

[oijauregui]

@bvdh here's the code to build R4 or R5 versions.
Still need to work on the possibility of running sushi . without calling the publisher.

[costateixeira]

Some comments / questions as I am personally interested in this and I understood a liquid-based substitution could work, but I can't figure it out:

• why committing the docker for the builder?
• If a custom script is really required (I hope we can work to avoid custom scripts, e.g. by incorporating some features in the standard scripts) how will this work in the CI pipeline?
• i see only .sh scripts. In principle we should not exclude windows users
  (Finally, I see a lot of content committed, I presume that content is not required for this change. e.g. template, R4 and R5 build....)

Is this the expected approach? Seems quite a lot and I don't know how we would standardize it, but I would like to aim for that.

[oijauregui]

Hi Jose,
Thanks for your comments.

• The docker file add liquid and other dependencies to the environment. It can be included in the startDockerPublisher.sh script, and get rid of that docker file, if it makes sense.
• Agree that is better to incorporate this funcionality to existing scripts. Which would those standard scripts to be modified in this case?
• About the Windows support, sure, once we agree on the functionality and directories structure I can work on replicating the functionalite in .bat files.
• I was not sure what to add to .gitignore , so I included the builds. Shall I leave them out then? The script creates the R4 and R5 directories structure on its own.