Migrate to PkgExtensions

[MasonProtter]

See JuliaFolds2/BangBang.jl#2 for an example of this and https://pkgdocs.julialang.org/v1/creating-packages/#Conditional-loading-of-code-in-packages-(Extensions) for the relevant documentation.

There's a number of conditionally loaded packages in Transducers.jl: https://github.com/JuliaFolds2/Transducers.jl/blob/master/src/Transducers.jl#L176-L196, and also some unconditionally loaded packages like Tables.jl that don't really need to be direct dependencies.

PRs migrating Transducers.jl to using the Extension system would be much appreciated.

[simsurace]

Seems the first part of this was solved by #5. The second part is still open, right?