feat(core): allow overrides to target formats without files

[SahilRajputdotexe]

Fixes #2446.

This lets an override select documents by formats instead of, or together with, files. The docs already list formats as an override target, but the schema required files and the format list was never looked at when picking an override.

An override now has to state files, formats, or both, and it applies only to documents that match every selector it states. A document whose format cannot be detected is not picked by an override that states formats, but still matches overrides that only state files. Formats that are only mentioned inside an override are used when detecting a document's format. To make that work, format detection now runs before the overrides are resolved. A document with no source is only an error when some override states files. Overrides that use JSON Pointers can state formats too.

extends: [spectral:oas, spectral:asyncapi]
overrides:
  - formats: [oas3_0]
    rules:
      operation-description: info

Only OpenAPI 3.0 documents get the changed severity. Other documents linted in the same run are not affected.

I added tests for the schema, Ruleset#fromSource and linting through Spectral#run. I also updated docs/guides/4d-overrides.md, whose example was not valid YAML.

Checklist

• Tests added / updated
• Docs added / updated

Does this PR introduce a breaking change?

• Yes
• No