Handle Web IDL

[annevk]

See whatwg/webidl#1018 for context.

Disclaimer: I haven't actually gotten npm run pp-webidl -- --input index.html to run locally yet.

[annevk]

One thing this fails to do is run npm install. Is this the first time that is needed? Should I just run npm install each time or do we want to handle that differently? @domenic?

[domenic]

I think the best setup would be to not run npm install as part of deploy.sh or the makefile, but instead have the CI build do the installation using @actions/setup-node and then an npm install step.

So, that'd require some additional work in .github/workflows, I guess :-/.

[andreubotella]

WebIDL's makefile was running webidl-pp for non-deploy builds, and its .pr-preview.json file had "post-processing": {"name": "webidl-grammar"}. The post-build steps will instead only run as part of make deploy. Now, the webidl-pp transformation isn't necessary on local or preview builds, since it will happen dynamically on page load anyway, but let's make sure that this isn't a difference that gets overlooked.

Also, there's the check-grammar.js script that makes sure the WebIDL grammar stays as LL(1). That should probably also run as part of the post-build steps.

[annevk]

I'm happy with this now.

I've not tackled .pr-preview.json. I did notice it, but I think at some point we probably want .pr-preview.json to use the output of the build process and not the output of Bikeshed/Wattsi. And as you note it is okay either way.

[annevk]

I'm not sure why I added the "do not merge yet" label, but I think this can be merged. I've also run it for whatwg/webidl#1018 and included the output therein.

I could change the node-version back to 16 if desired. I noticed we use 14 elsewhere which is why I went with that here. Up to @domenic.

Note that this PR now also adds the ability to run factory.py without generating PRs and with the ability to create changes for a single repository, which is useful for things that are being transitioned.