Accept `?wa` as a shorthand to `?backend=wa`.

`jscoq.js` is needed.

In scratchpad only.
Closes #326.

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.

There is no longer @jscoq / @wacoq separation in pkgs.

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...

The bundle (`.bundle.min.js`) is now needed for the menu.

To avoid confusion, `ui-js/jscoq.js` has been renamed.
Also, a legacy-mode `jscoq-loader.js` that emulates the old behavior.

So that it can be `import`ed.

Maybe we should squash the loader fixes to the previous commits.

We use a strategy top-down to convert to modules. For now we convert
the loader to a module. This requires a few minor changes, but
shouldn't be too hard.

Sync `package.json.wacoq` with `package.json`.

And immediately employed this feature in the sqrt_2 example.
The example now contains some hand-crafted prologue followed by the
coqdoc content.

Instead of sqrt_2, the landing page shows a basic proof with lists.
Need to make a wider portfolio of proofs from different fields as
discussed on Zulip.

Instead of sqrt_2, the landing page shows a basic proof with lists.
Need to make a wider portfolio of proofs from different fields as
discussed on Zulip.

Ordering of results in completion hints, collapsing n/a results, scroll shadow.

`<kbd>` formatting and a hint for newcomers to look at the scratchpad icon.

`<kbd>` formatting and a hint for newcomers to look at the scratchpad icon.

Filled in the links in the navbar, etc.
Added hover action to quick-start reference.

Added a link to Coq in "settings".
The top-left link now points to jsCoq's homepage instead.
Landing-page CSS moved to a separate file.

As suggested by @hannelita.

Thanks @mdnahas :D

Can now return to the original `Import` statements.

Keyboard slip caused it to revert to the old Gitter url.

Fav icon is now PNG.

Keyboard shortcuts are added via a CodeMirror.keyMaps entry.
In literate mode (snippets), PgUp/PgDn are not bound, to allow scrolling.