Support `Base.emptymutable`
I wonder if we could add support for Base.emptymutable in StructArrays? For arrays it behaves like similar but with guarantee that you can push!, etc to the output (while similar outputs must support setindex! but not necessarily resizing), and is used internally in various algorithms (and occassionally in the package ecosystem, e.g. xref https://github.com/JuliaData/SplitApplyCombine.jl/pull/54).
It makes sense to add it, but maybe it's best to wait to get some confirmation at https://github.com/JuliaLang/julia/issues/46230 that it's actually a stable API?
~I've updated https://github.com/JuliaLang/julia/pull/49958 to use similar in emptymutable. In that case, we won't need to extend it here.~
Edit: emptymutable isn't added there anymore