chore: add gemini streaming example (#782)

examples/gemini/streaming.ts

+import { gemini, GEMINI_MODEL } from "@llamaindex/google";
+
+(async () => {
+  if (!process.env.GOOGLE_API_KEY) {
+    throw new Error("Please set the GOOGLE_API_KEY environment variable.");
+  }
+  const llm = gemini({
+    model: GEMINI_MODEL.GEMINI_PRO_LATEST,
+  });
+  const stream = await llm.chat({
+    messages: [
+      { content: "You want to talk in rhymes.", role: "system" },
+      {
+        content:
+          "How much wood would a woodchuck chuck if a woodchuck could chuck wood?",
+        role: "user",
+      },
+    ],
+    stream: true,
+  });
+  for await (const chunk of stream) {
+    process.stdout.write(chunk.delta);
+  }
+  console.log("\n\ndone");
+})();