migrate towards using a monorepo: ES6/rollup/etc. is a horror otherwise

[GerHobbelt]

See also: https://github.com/GerHobbelt/jison/projects/3

Now that I am upgrading jison et al to produce ES6/ES2015 compatible output and be ES6/ES2015 itself (at least as far as modules' import/export is concerned), the split into submodules proves to be a real horror as it is NOT a dependency tree but rather a (cyclic!) dependency graph at development time.

In short: 💀 💢 😭 👿 👎 👎 👎

[GerHobbelt]

Done. (Well.... sort of. The subpackages are still a hassle to publish but at least jison itself is now produced much more easily and with no risk of screwing up the rebuild process.