CompatHelper: bump compat for BangBang to 0.4, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the BangBang package from 0.3.28 to 0.3.28, 0.4. This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.

[ChrisRackauckas]

@MasonProtter is anything blocking this?

[MasonProtter]

This is the old repo, the actual Transducers repo already is on BangBang v0.4: https://github.com/JuliaFolds2/Transducers.jl/blob/master/Project.toml#L44

[ChrisRackauckas]

ahh I see. https://github.com/JuliaFolds2/Transducers.jl/pull/38 is what it was.

[MasonProtter]

Ah gotcha. Tagging a new release now.