Validate that path parameters have schema or content

[colinmollenhour]

User story.
As a user, I would like Spectral to warn me when I forget to specify a schema or content in a path parameter.

Is your feature request related to a problem?
Yes, I forgot to add a schema and it resulted in much confusion as Spectral passed validation but other tools choked with unhelpful error messages and it took a long time to realize a simple mistake.

Describe the solution you'd like
If I forget to specify a schema or content for a path parameter, Spectral should warn me with an error like:

Parameter "${p}" must have either a "schema" or "content".

Additional context