stacker dependency optional (enabled by default)

[amascolo]

The stacker crate isn't strictly needed.

We may not wish to use it in production.

Also, its dependency psm doesn't support WASM.

It can be made optional (enabled by default).

[CrockAgile]

This seems reasonable to me! stacker is only used for generation, which is only a part of BNF.

[coveralls]

Coverage: 92.231% (+0.1%) from 92.114% when pulling ab24b96 on tably-team:main into f3ca021 on shnewto:main.