add keyword index and retriever

packages/core/package.json

packages/core/src/Node.ts

packages/core/src/Prompt.ts

packages/core/src/indices/BaseIndex.ts

packages/core/src/indices/keyword/KeywordTableIndex.ts

...es/core/src/indices/keyword/KeywordTableIndexRetriever.ts

packages/core/src/indices/keyword/index.ts

packages/core/src/indices/keyword/utils.ts

pnpm-lock.yaml