feat(oas): validate 'default' in schemas

[P0lip]

Closes #1402

Checklist

• Tests added / updated
• Docs added / updated

Does this PR introduce a breaking change?

• Yes
• No

Additional context

I'm very torn when it comes to the solution chosen here.

Besides what's implemented here, I considered the following:

• 2 completely new rules called oas2-valid-schema-default and oas3-valid-schema-default - this might cause redundant fragmentation
• renaming existing rules to something more generic, i.e. oas2-valid-schema (wouldn't it be confusing, though?) - this one would be breaking and therefore defer the release of the fix

I still think that ideally we should rename the rules, so that we can validate more things in the future using the same rule.
I'm not fully certain, however, so hm... @philsturgeon ideas?

[philsturgeon]

I think its ok to have it in default for now, its really similar. Only example and default work this way, so making a more generic oasX-valid-schema would just end up causing future bloat I think. If people report being confused by this we can split the rules but there's far bigger fish to fry I think.