Simple mechanism to share code between browser-specs and Reffy?

[tidoust]

There's a bit of duplicated logic between browser-specs and Reffy:

• the method that loads a spec with Puppeteer (although there are a few nuances on that one)
• the throttled queue, which could end up being useful in other projects as well.

It would be good if we could share the code instead of duplicating it. Creating an NPM package is a bit overkill and would create maintenance overhead. We could perhaps import the code directly though, e.g. by switching to ESM modules.