build: add lexgen as go tool #918

TIL the tool section on go mod files! unfortunately we do not use lexgen from upstream indigo, we use a fork that i maintain here: https://github.com/oppiliappan/indigo which produces slightly different codegen. we also do not recommend using lexgen as-is, but rather the flake app nix run .#lexgen, which has some nice conveniences.

i'd prefer if devtooling were provisioned my nix and not go.mod.

huh meant to close that PR, not merge it. oops.

ah ok! that would explain why I had trouble getting the same generated files in my other PR. Gotcha!