Because of the way Odoo works at its core, we do not know before hand
which files will be loaded as an asset in the browser, because it
depends on the installed Odoo addons.  This is why it is historically
difficult to integrate Odoo with standard JS tooling, and this is why
Odoo needs to use a custom javascript module system.

However, there is a way to use native JS modules (and gain all the
benefits from it: IDE autocompletion, ease of refactoring, intellisense,
...): we can write JS as native JS modules, but convert them at runtime
into Odoo custom modules. This is exactly the strategy applied by this
PR.

This has a lot of benefits, but there is a downside: we can no longer
serve statically JS files in debug=assets.  This would be a dealbreaker,
if we did not have sourcemaps.

So, this PR introduces

- support for native JS modules
- add a specific asset controller `/web/assets/`
- generate sourcemaps (for debug=assets mode)

Note that part of the complexity of this work is cause by the debug=assets mode, where we need to have a fixed route for the debug asset bundle (so we do not lose breakpoints when debugging), but we still need to safely regenerate the bundles whenever the JS code is modified.