Messaging for missing source maps

Part of #10369

We want to encourage users to make source maps available by enticing them with what's possible in the report. There are a few approaches to consider:

1) top-level warning

Too prominent. Listing to rule it out.

2) audit-level warnings

For audits that require maps to do anything useful (ex: bundle duplication), the audit should have a warning. This should expand the audit my default and list it right after the Failing audits but before the Passed audits. The audit can't be marked N/A (otherwise it wouldn't be in the warning clump). Should it be marked as passing?

For audits that don't require maps but are improved by them, a warning is almost what we want but may come off as too loud. What if we added a "notice", like warning but they don't get added to their own clump like warnings do?

3) there isn't a third option, maybe suggest one?

i started listing things but ran out of things at 2 ...

4) missing-source-map audit

Oh, I remembered another.

@paulirish suggested an audit that fails if a first party JS file is >x KB (resource size) and doesn't have a map. x = 500 KB? Over time we may reduce this threshold.

Alternatively this could be added to the still-WIP `valid-source-maps` audit. The plan was to just make it informative (ex: maps are found but mappings seem invalid, not a huge deal but would make code mappings inaccurate), but it could also fail based on the above criteria.