Schema context accessors and all servers in Document

Additions

• Adds allServers on OpenAPI.Document to retrieve all servers that are referenced anywhere in the document (whether in the root server array, the servers on a Path Item, or the servers on an Operation.
• Adds accessors that retrieve contexts on JSONSchemas, providing an alternative to destructuring when retrieving an optional context is more convenient.

Bug Fixes

• Breaking: Fixes return type on Components.forceDereference() to be non-optional. It was made optional when forceDereference() was introduced in v1.2.0 by accident even though it is not possible for that function to produce an optional value. I decided this was an isolated enough breakage to something introduced very recently and therefore am not waiting for a major version to fix the bug.