Es Modules in the runtime?

[arthuro555]

Should we consider a way to use es modules in the runtime? They have some advantages:

1. They are nice to work with
2. They seem to be where the future typescript/javascript is headed towards, with ts discouraging the use of namespaces
3. They allow importing npm modules directly in files (something we cannot do with namespaces)
4. They would probably solve some of our typing issues with PIXI

And, if we go as far as to convert the whole Runtime to es modules, we could gain tree shaking, which could allow to remove dead code making the games less bloated and therefore faster, especially when using a few features of a lot of extensions. Though, this would mean a full rewrite of the code generator and adding something like esbuild to the GDevelop build pipeline (that should be pretty easy to do and not impact too much the performance though).

We can also try to add support for writing an extension with es modules, by only building index.mjs if one such file is present in the extension root directory and building it in esbuild with the option bundle: true, though that wouldn't let us profit from the really interesting points like tree shaking or resolving PIXI typing issues, it would allow using and updating npm packages more easily.