Gatherer/artifact cleanup

[paulirish]

In a plugin world, the gatherers become part of our public API. We know about a few things we're interested in changing. Let's capture them here.

• add a new MetaElements gatherer which gets name/content of all of them. This would nullify the following: ThemeColor, Viewport, MetaRobots, MetaDescription, (core(meta): refactor all meta audits to single artifact #7025)
• add a new LinkElements gatherer (..actually introduced in core(preconnect): add warning if preconnect link is not used #6694 as ExternalResourceLinks). This could nullify Canonical and Hreflang (core(canonical): move canonical audit to LinkElements #7080)
• add a new AnchorElements gatherer which gets rel/href/target/text of all a elements, nullifies AnchorsWithNoRelNoopener and CrawlableLinks (core(artifacts): unify AnchorElements into single gatherer #7101)
• convert ImageUsage into ImageElements gatherer (core(images): cleanup ImageUsage to match other *Elements artifacts #7030)
• convert Scripts into ScriptElements gatherer (core(scripts): move to ScriptElements artifact #7920)
• ...

@patrickhulce you had a few in mind, too, right?

[patrickhulce]

meta and links were the two big ones 👍 I also wanted to cleanup

• Images into ImageElements
• AnchorElements

[patrickhulce]

I like the standardization on *Elements I'll update #6694, my only concern with LinkElements was that it would be easily confused with a elements :) I think it makes sense though when they'll be side-by-side

[egsweeny]

Is this issue comprehensive with what we're wanting to accomplish with artifact cleanup, or are there more that need to be tended to?

If it's not the full list, then can Connor take on fleshing it out?

[patrickhulce]

I think the primary goal is getting our artifacts in a state ready and useful for plugins. If someone wants to do some brainstorming what audits might need to know about a page and how we should standardize it, that'd be super useful :)