Switch to esbuild for building the library

[oleiade]

This PR drops webpack and switches to esbuild to build and bundle the library.

Here are a couple of reasons for that:

• esbuild is much, much faster than webpack.
• esbuild feels simpler to configure (at least to me), and allows for a cleaner, more maintainable configuration. It took me a day to configure webpack+babel, and I was not certain I understood what was happening. Esbuild took me 30 minutes, and I do understand what's going on.
• we already rely on esbuild to support typescript in k6, and aligning things to gain experience and confidence around it made sense.

Apart from switching to build, the most notable change in this PR is that we configured it to use the ES modules format instead of CommonJS. The recent introduction of native ES Modules support in Sobek shouldn't pose any problem, but you never know.

[oleiade]

Apologies for the noise, switching to draft as I caught issues in CI after the fact 👍🏻

[oleiade]

I'm so confused, but TIL that if you delete a branch, even mistakenly, it automatically deletes the PR...