feat(ls): fix completion paths, self and indirect refs

[frantuma]

fixes completion of references:

• doesn't include reference to self
• fixes JSON Pointer paths escaping
• doesn't include by default indirect references (target nodes being a reference themselves).
  Provides a completionContext option includeIndirectRefs: if set to true indirect refs are also included, but only when the document namespace (specification + version) allows for siblings of $ref field, as this is the only case where such a reference would make sense. Currently the only namespace supporting this is OpenAPI 3.1