[Feature Request] Expose ServiceContext.ts as a separate package

[AndreMaz]

ATM the contents of ServiceContext.ts are only available by installing the llamaindex package. However, this significantly increases the node_module size and causes deployment issues (see: #1572 and #1580 (comment))

Current behavior

export { serviceContextFromDefaults } from "llamaindex";
// OR
export { serviceContextFromDefaults } from "llamaindex/ServiceContext";

Expected behavior

export { serviceContextFromDefaults } from "@llamaindex/<SOME-PACKAGE>";

Same thing happens with the VectorStoreIndex that's only available via

export { VectorStoreIndex } from "llamaindex"
// OR
export { VectorStoreIndex } from "llamaindex/indices/vectorStore/index";