- don't rebuild the cli stuff when the frontend/backend changes
- make tests work again

This is not perfect but it works (and it is much faster).

For some reason I couldn't make `fflate-unzip` work with ESBuild, so
I've placed a copy and tweaked the exports.

We reorganize jsCoq's source code as to be more modular, in particular
we start to place backend files under the `backend` directoy, and
frontend files under `frontend`.

Code as it now does work, note however that still quite a bit of work
is needed. In particular, the headless frontend should get its own
directory, the coq-jslib directory should go away, use more types,
etc...

Still, both don't fully work, needs more research as to witness what's
broken.

Must be set this way in 8.13. Will revise in 8.14 using the improved
`Stm.new_doc`.

In cases where the element has a padding, as is the case with `div.coq-env`.
The unit-test gallery has been updated to reflect this situation.

Trying to be more faithful to what `Format` does:
- take into account offset parameter defined on boxes.
- adhere to left margin of containing box, when breaking lines.
- be more declarative via CSS.

I think this would handle #126.

Each snippet is now shown both before and after
break adjustment.

In pretty-printing of goals and terms.
Not perfect, but it's progress.
Also, there are three unit tests now. :P

Upgraded mocha and switched to async version of child_process.

What was needed in order to run unit tests in Node.js is
now located in `headless.ts`.
It is invoked via the regular CLI (bundled with Webpack).

Fixes #195.

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