Proposal: remove the need to rely on webpack for es5 worker generation

For purposes where we need to generate a bundle (the demo, the worker
files, the RxPlayer bundle builds), we're progressively removing our
reliance on webpack toward esbuild.

It began with the demo page originally for performance reasons, but we
found out that the esbuild behavior and API was much simpler to
understand than webpack's, and so we ended up also prefering to rely on
it for our worker build.

Considering that we since the v4 only rely on TypeScript with no
bundling step for our "regular" build (and our home-made scripts in the
`./scripts` directory), we thus ended up having no reliance on webpack
for most of our builds.

However at #1400, we noticed that we might also need to provide an ES5
version of our worker file (at least for the PlayStation 4), and I found
that having ES5 support through esbuild is not straightforward
(evanw/esbuild#297).

Historically, [babeljs](https://babeljs.io/) was used to translate to
ES5 - and that's the one I know, so I tried using it on the bundle made
by esbuild.
However it turns out that there's no resource for how to use babel
like this - not integrated as a bundler's plugin (here we just wanted to
translate an ES2017 already-made bundle to es5) so I just gave up and
relied on webpack for specifically that ES5 build.

However this both added some perceptible delay in our build, and we
now relied on two different bundlers in our `npm run build` main
build script, which made me un-comfortable in terms of maintainance and
understanding of how our build step worked.

So I re-looked around and read about [`swc`](https://swc.rs/), which
seems to be some kind of babel competitor with some credibility (though
they advertises themselves as a """platform""", I would have preferred a
clearer term but ok), with some differences, including speed, but what is
most interesting here is that it can just be run as a standalone
transpiler (what I mean: not embedded in a bundler).

By using it, I've been able to remove reliance on webpack (and babel) in
our regular build steps (produced by `npm run build`) which should be
realistically used by 99.9999% of applications (approximately!). The
only exception where both webpack and babel are still used is to produce
our bundles attached to each release notes (they expose the RxPlayer
through `window`, like our grandfathers did) - which I guess are rarely
relied upon.
Still, we should probably remove reliance on webpack there in the
future.

Swc is maintained apparently by volunteers, and is less known than
babeljs, so there's still that. Also, the bundler and transpiler
JavaScript landscape is __VERY__ active (even in JavaScript terms) right
now with projects like [rolldown](https://rolldown.rs/) and
[oxc](https://oxc-project.github.io/) so sadly it may be not the last
update of this part of our build process.