Review Node.js modules bundling instructions

[heitortsergent]

Related to #1611.

We have a set of instructions in the docs to help users learn how to bundle Node.js modules so they can use it with their k6 tests.

The JS landscape has changed quite a bit since that was first written, so we should review the instructions to make sure they still make sense (I recently tested them and the Webpack steps still work) and that we recommend the best path for users.

Some steps:

• Review bundler options
  • Add mention of https://esbuild.github.io/
  • Add mention of grafana/k6pack
• There's some confusion in the Webpack example about creating a file named "some.service.js" or "signup.test.js"
• Review how k6 TypeScript and esbuild support impacts these instructions