Remove TransformVariables.

[tpapp]

Motivated by discussion in TuringLang/Turing.jl#1877, the TransformVariables-related code will be moved to https://github.com/tpapp/TransformedLogDensities.jl. This makes this package lighter, getting closer to the goal of an interface definition package after AD code is removed.

TODO:

1. update readme once the abovementioned package registers
2. deprecations
3. bump version, release, describe migration for users, update docs
4. excise relevant code from TransformVariables -> will be done in relevant repo
5. updated examples for DynamicHMC -> will be done in relevant repo

[devmotion]

I think that's the simpler and better approach probably.

[tpapp]

@devmotion, what would you suggest for deprecation? Eg

1. @deprecate stuff moved to another package in a patch version, then remove in a minor version? or

2. just announce the change without @deprecate?

[devmotion]

I think deprecating (probably with Base.@deprecate_moved?) has the advantage that there is a clearly communicated, officially recommended way of how to upgrade to TransformedLogDensities. So downstream packages that run tests will see the deprecation warnings and how fix them, whereas otherwise they have to dig through PRs and/or release notes. So I guess basically I share Invenia's view on deprecations in Julia: https://invenia.github.io/blog/2022/06/17/deprecating-in-julia/