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...

We move vendored things to `vendor` and let dune know about it.
We bump serapi dep to the head of the v8.16 branch.

We also adjust build deps a bit better, in particular to avoid Dune
3.4 which is buggy for us.

Beware this will require `git submodule update` on the work tree.

- Migrated patches for 8.14.
- Up wacoq to 8.14 in docker image
- install: 8.14 requires `coq-core` and `coq-stdlib` to be installed.
- Set version 0.14.0.

Nothing too remarkable, code-wise, but a big change upstream is the
addition of `zarith` as a hard dependency.

TODO:

- update addons

Changes in code are minimal. Additionally:

- we bump SerApi dep to 8.11
- implement a few primitive float functions so Coq's kernel doesn't
  complain on startup.

We depend on serlib to serialize Coq data structures and remove the
duplication present in jsCoq. This greatly expands the range of Coq's
types available for sending as JSON.

Next commit will add a query to showcase UI grabbing the full AST for
a sentence, which allows pretty cool things UI-wise.

For now, we include serlib as a module, which is a bit cumbersome,
however once we release serapi 0.7.0 we should be able to just depend
on the coq-serapi OPAM package.

Closes #89

- we don't doubly-build PRs coming from my own branch
- no need to use OPAM sandboxing in CI builds
- check npm dependencies too
- remove CodeMirror submodule in favor of npm

CodeMirror-TeX-input won't be merged in main CodeMirror thus is a new
external dependency.