Adds a new `decl` case: Ignore
Adds a new `decl` case: Ignore

This case is used in auto mode to expose directly generated functions
t_of_js/t_to_js from a type t.

Before it was implemented using a `Cast` which was hard to extend
to support polymorphic types.

LexiFi/gen_js_api,alainfrisch/gen_js_api,LexiFi/gen_js_api