Core PR 20 changed \ to accept either:
- A single JSON Pointer string (existing behavior)
- An array of JSON Pointer strings for multiple inheritance

Multiple inheritance follows first-wins semantics - properties from
earlier types in the array take precedence over later ones.

Changes to all SDK validators:
- Schema validators: Added validateExtendsKeyword() method to validate
  both string and array of strings
- Instance validators: Updated to handle array of \, merging
  properties from each base type in order
- Reference rewriting: Updated rewriteRefs to handle \ arrays
  when processing imports

Also added a new sample demonstrating multiple inheritance:
- sdk/primer-and-samples/samples/core/12-multiple-inheritance/