Multiple YAML documents per file / NDJSON

Nobody else seems to have discussed this, but it seems to be impossible `ajv-cli` to validate YAML files that contain multiple documents. My underlying goal is replacing `RoadieHQ/backstage-entity-validator` with more pure AJV, because BEV does not permit passing many of the AJV options.

The validator seems to throw strange errors
```
error: Invalid left-hand side expression in prefix operation
error: Unexpected identifier 'REDACTED-INTERNAL-STRING'
error: Unexpected token 'for'
```

I don't want to split the documents up into multiple files because I'd like to return original document line number references to the errors, to help end-users fix their documents.

`data/schemas/allOf.v1alpha1.schema.json` contains an `allOf` array w/ if/then conditionals to select between different schemas based on a value in a top-level key (it's not the name of the top-level key, so I can't use `discriminator`). The globbing hack is to avoid ingesting the `allOf` schema twice.

```
ajv validate --all-errors --spec=draft7 -r 'data/schemas.upstream/**/*.schema.json' -r 'data/schemas/!(allOf.v1alpha1).schema.json' -s data/schemas/allOf.v1alpha1.schema.json --validate-formats=true --strict=log -c ajv-keywords -c ajv-formats --verbose --data --errors=text -d $DOCUMENT_YAML
```