New in 8.17.

Instead of `.cjs`; enough with this nonsense.

These are the current numbers on release:

```
 261K -- dist/wacoq_worker.js
 369K -- dist/frontend/index.css
 583K -- dist/frontend/index.js
```

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

Seems to work! I guess it can be done better, but for now that is OK.

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.

This is a large build-time improvement, moreover we can get rid of a
lot of troublesome code regarding how to manage assets.

`esbuild` has proven a great tool so far.

Tested with both backends.

We still keep the CLI and the WASM backend on webpack, the idea is
to move them to esbuild in subsequent commits.

CLI should use a native Coq instead, it is super-slow.

We also disabled the `collab` and `ide-project` builds for now,
re-adding before merging hopefully.

The compatibility story with user .html pages needs to be revised,
IMHO as this is a foundation for the LSP backend this is the time to
drop support for some patterns we had in the past.

Borrowed from #275

All wasm binaries are read from `node_modules` directly, so there is no
need for a `bin/` directory (this was a hack).

The deps are collected in a meta-package `wacoq-deps`, which is
published to NPM separately and should not change often.

Added exe mode to wacoq by retrieving `wacoq_extern.c` from wacoq-bin.

The wasm backend bytecode is now loaded
from `backend/wasm` (and also linked
accordingly).

Dynamic linking is moved to jscoq_worker since it is only relevant there.
(In wasm, native dynload is used.)

JS backend works.

Removed the backporting discrepancy in package builder.

Will gradually move parts from the other repo, wacoq-bin, to `backend/wasm`.
This will allow some code reuse between the backends.

We also move from hacks to a proper config object (which can be
properly created in a dynamic top-level context).

Also, note that we now use a ES6 map for the routes, this is more
convenient to add / remove elements than using the bare object.

This is rarely used live, and should allow us to make progress towards
the common protocol a bit more easy.

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...