Add a global ReSpec API

[sidvishnoi]

document.respec = {
  // same as window.respecVersion
  version: string,
  errors: ReSpecError[],
  warnings: ReSpecError[],
  // same as document.respecIsReady
  ready: Promise<void>,
  // catch ReSpec errors/warnings (extends EventTarget)
  addEventListener: ("error" | "warning") => void,
  // export static HTML
  toHTML: () => Promise<string>,
}

• document.respec basic (https://github.com/w3c/respec/pull/3082)
• document.respec.ready (https://github.com/w3c/respec/pull/3082)
• deprecate document.respecIsReady (https://github.com/w3c/respec/pull/3090)
• document.respec.toHTML() - Part of https://github.com/w3c/respec/issues/2889 (https://github.com/w3c/respec/pull/3403)
• document.respec.errors[], document.respec.warnings[] - Closes https://github.com/w3c/respec/issues/1084 (https://github.com/w3c/respec/pull/3348)
• document.respec.addEventListener("error" | "warning") - Part of https://github.com/w3c/respec/issues/3035 (https://github.com/w3c/respec/pull/3338)

[dontcallmedom]

I'm not sure if it makes sense in this context, but one feature [I needed recently]( which could be hosted in this global API: in a postProcess script in ReSpec, I wanted to use data from the WebIDL parser, but there is no way to access the parser or the parsed IDL tree right

[sidvishnoi]

I'm not sure if document.respec would be the right place to expose this as I would like to keep the API surface as small as possible here to avoid any breaking changes.
FWIW, we do have plans for a new plugins API (See https://github.com/w3c/respec/issues/1976#issuecomment-659673129), where it might be easier to expose this functionality.