feat(js): Expose light interface for tracing client

[jacoblee93]

Allows for interoperability in LangChain when initializing tracer with clients if different LangSmith versions are resolved:

import { Client } from "langsmith";
// `@langchain/core` may have a different internal version of `langsmith`
import { LangChainTracer } from "@langchain/core/tracers";

const client = new Client();

const tracer = new LangChainTracer({
  client, // Will type error if resolutions are bad
});