* chore: enforce file extensions on imports
To make our output agnostic it should include file extensions in the
output, as per the ESM spec. It's a bit odd for Node packages but makes
it easier to publish a browser build.
DOMWorld only needs to use Node's `fs` module if you're adding a
filepath as a script/style tag. We can detect this case and run the
`require` inline such that in a browser this code won't execute.
These files will be used by both the web and node versions of Puppeteer.
Another name for this might be "core" but I don't want to cause
confusion with the puppeteer-core package that we publish at the moment.
