add deletion/movement-quasimode for selected node in variadic list

[disconcision]

the basic case here shouldn't be too bad.

1. make sure holes spawned in variadic forms are flagged at creation as being members of variadic lists, and that this flag properly carries through transformation

2. write some transformations to delete, insert, swap-left, and swap-right.

3. bind to ins, del, C-left, C-right.
   (actually, maybe C-ins/del as well? consistency, makes deletion more purposeful)

insert: new hole before? selection
insert-end: coax the variadic list terminal + into producing an extra terminal hole.

question: make this into a 'proper' quasimode? like, make everything use C-, and do some mode-related color change?