feat: add vercel chat example (#1532)

.changeset/clever-monkeys-switch.md

apps/next/src/content/docs/llamaindex/integration/meta.json

apps/next/src/content/docs/llamaindex/integration/vercel.mdx

examples/package.json

examples/vercel/README.md

examples/vercel/llamacloud.ts

examples/vercel/vector-store.ts

packages/providers/vercel/package.json

packages/providers/vercel/src/index.ts

packages/providers/vercel/src/tool.ts

packages/providers/vercel/tsconfig.json

pnpm-lock.yaml

tsconfig.json