Move the /build to a build step instead of versioning it in the repo

[jasonbahl]

What problem does this address?

Currently we are versioning the build output for the GraphiQL IDE and other JS assets (the extensions page, etc)

There's no reason to version these assets in the repo.

What is your proposed solution?

We should move this to a build step when the plugin is being built for deploying to WordPress.org, etc.

What alternatives have you considered?

Leaving it as-is.

Additional Context

I'm not 💯 sure how this would impact folks that install via composer?

[justlevine]

Additional Context:

• We're already building in our release CI, so not much needs to change there.
• Only 'disruptive' change is users who are still checking out GH or the source tree.
  • Good opportunity to refactor script loading and improve the error messages when they're not built.
  • we could add a post-install-cwd to Composer that's npm ci && npm run build, so long as it doesn't leak into wpackagist.