Support multiple Pony compilers

At the moment, we build documentation using "the latest release" Pony compiler. "The latest release" assumes we have updated the Dockerfile we use. Each time we create a new main.actor CI container, it will use whatever the latest Pony release is.

We want to be able to support building docs that require different versions of the Pony compiler. This involves a few things...

The ability for a library creator to indicate which version of Pony a given release should be built with

@jemc and I have talked about this and we think that this should probably be information that is in tack.json as its a dependency issue and is something that tack is eventually going to probably grow into needing.

Having more than 1 pony compiler available in our container
The ability to easily switch "the active" pony compiler

Neither of these 2 issues needs to be addressed until the first part of this issue is addressed.

ponylang / main.actor.archived

Support multiple Pony compilers #1