feat(extractors): add keyword extractor and base extractor (#404)

.changeset/three-hounds-count.md

examples/extractors/keywordExtractor.ts

examples/extractors/questionsAnsweredExtractor.ts

examples/extractors/summaryExtractor.ts

examples/extractors/titleExtractor.ts

packages/core/src/Node.ts

packages/core/src/extractors/MetadataExtractors.ts

packages/core/src/extractors/index.ts

packages/core/src/extractors/prompts.ts

packages/core/src/extractors/types.ts

packages/core/src/index.ts

packages/core/src/tests/MetadataExtractors.test.ts

packages/core/src/tests/utility/mockOpenAI.ts

packages/core/tsconfig.json