Enable generators by default (even in dependencies)

[anmonteiro]

fixes #87

For projects that want to check in generated code, setcut-generators: false in bsconfig.json

[jfrolich]

Pardon my ignorance. But what are generators in this context?

[jchavarri]

@jfrolich it's this feature that allows you to define custom rules that will be bridged to ninja. Somehow analog to dune rule stanza. Useful to generate ml files with tools like menhir or atd. I can't find it in new rescript docs but you can see the old documentation in https://bucklescript.vercel.app/docs/en/build-advanced.