Restore MCMCChains for MCMC diagnostics and plotting #2898

MCMCChains still needs better native support for DynamicPPL.VarNamedTuples, which emerged from the recent refactoring of DynamicPPL’s tracing and VarInfo machinery. That is, however, a fairly contained integration issue: the diagnostics and plotting machinery remains useful and is still relied on elsewhere in the ecosystem.

See, e.g., TuringLang/MCMCChains.jl#511 (comment) for related discussion.

For some background on how VarName and VarNamedTuple evolved, see the VarNamedTuple work in TuringLang/DynamicPPL.jl#1150, TuringLang/DynamicPPL.jl#1180, TuringLang/DynamicPPL.jl#1181, TuringLang/DynamicPPL.jl#1182, and TuringLang/DynamicPPL.jl#1183, along with the earlier VarNamedVector work in TuringLang/DynamicPPL.jl#1098.

The underlying design discussion goes back to the early days of Turing. The need to identify subsets of model variables for inference was already noticed in the 2016 interface discussion at #55, and by 2017 VarName and VarInfo were established enough that their place in the public interface was being reconsidered in #226. That lineage later led to an RFC proposing structured VarName indexing rather than strings in TuringLang/DynamicPPL.jl#49, implemented in TuringLang/DynamicPPL.jl#50; see also TuringLang/DynamicPPL.jl#528, #2423, and TuringLang/DynamicPPL.jl#416.