Create dev build infrastructure

[handrews]

I had a check fail on #4188 on v3.2-dev and realized that there's no way to build without copying things over to the publish location (and even then something went weird when it was trying to build the old versions b/c the dev branches aren't set up for that).

We should be able to build just src/oas.md for testing purposes on dev and all branches derived from dev.

This change should go into dev and be merged/cherry-picked out from there.

If Arazzo and Overlay are going to use the same system, this might also be a good time to start building a shared system instead of continuing to patch things in triplicate.

[ralfhandl]

@handrews How luxurious do you want it to be?

The simplest solution would be to add a script npm run build-src that creates a static oas.html next to the oas.md (or anywhere you like) and add that file to .gitignore.

A nicer experience would be to start a tiny express server via npm run serve-src that dynamically produces an ephemeral oas.html on every change of oas.md and tells the browser to auto-refresh. That way you could have a browser window next to your editor window, type in one and live-check the other.

I have copy templates for both approaches lying around 😎

[handrews]

@ralfhandl I'm perfectly happy with the simple version (and would be more likely to use it TBH because all of my dev tools tend to be the lowest-tech that works for me). But if you want to do the more complicated thing I'm not going to stop you.

Although... express seems like a bit dependency to add, and I'd kind-of like to be able to just build and look at the HTML without running a server. I guess just make sure I don't have to understand express at all to use the build system.