start of basic pdf & directory readers

packages/core/package.json

packages/core/src/readers/PDFReader.ts

packages/core/src/readers/SimpleDirectoryReader.ts

packages/core/src/readers/base.ts

packages/core/src/storage/FileSystem.ts

packages/core/src/storage/kvStore/SimpleKVStore.ts

packages/core/src/storage/vectorStore/SimpleVectorStore.ts

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

pnpm-lock.yaml