Change bundle tool to Rollup and gen the esm

[Aloento]

I haven't read your entire project, so you may need to make some changes to fit your reality.

[elringus]

Hi, Thanks for the PR, but I'm not comfortable with rollup and would prefer using webpack for the time being.

[Aloento]

in fact, it's esbuild.
https://github.com/Elringus/DotNetJS/blob/847b913bee7cceb2b7b7544bd80b9eedb06615c3/JavaScript/dotnet-runtime/rollup.config.js#L5
and the product doesn't have anything like this:

so you are be able to make change.
and I fixed the "this / self / globalThis" issue.

[elringus]

Thank you, this will be helpful in case we'll decide to switch to rollup (which looks like the current standard for ES module building). I'd like to first wait for .NET 7 release (which promises some modularization feats) and see if we can drop the custom branch. After that I'll look into migrating the library to ES.