allow to set fontc --emit-ir=false

[anthrotype]

Fixes #224

credits to @rsheeter for the hint to using ArgAction::Set

clap-rs/clap#1649 (comment) might offer a path

[anthrotype]

hm why am I getting:

the trait bound SmolStr: Serialize is not satisfied

https://github.com/googlefonts/fontmake-rs/actions/runs/4567180455/jobs/8060649666?pr=225#step:4:176

when I run my pre-push git hook before pushing, it did not catch this?!
Similarly if I cargo clean just to be safe, and then I run cargo test --all-targets --all-features like the CI also does, it PASSES on my local machine... 😕

[anthrotype]

turns out, I had to run cargo update so that my local Cargo.lock would get updated (the CI runs from clean directory and has no Cargo.lock so fetches whatever is available), and then I was able to repro the error locally with smol_str = "1.1.25". Apparently the serde feature is now optional in the latest version so must be declared explicitly.

[rsheeter]

Approving to see if I can reproduce a GH bug. Also I'm fine if you merge this when ready :)