feat: abstract file system api (#433)

packages/core/src/embeddings/utils.ts

packages/core/src/env/index.edge-light.ts

packages/core/src/env/index.ts

packages/core/src/readers/CSVReader.ts

packages/core/src/readers/DocxReader.ts

packages/core/src/readers/HTMLReader.ts

packages/core/src/readers/ImageReader.ts

packages/core/src/readers/MarkdownReader.ts

packages/core/src/readers/PDFReader.ts

packages/core/src/readers/SimpleDirectoryReader.ts

packages/core/src/storage/FileSystem.ts

packages/core/src/storage/StorageContext.ts

packages/core/src/storage/constants.ts

packages/core/src/storage/docStore/SimpleDocumentStore.ts

packages/core/src/storage/indexStore/SimpleIndexStore.ts

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

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

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