feat: Vercel AI SDK v6 with tool-calling

[jakobhoeg]

This PR adds a Next.js example application of how to use Transformers.js models that support tool-calling with the Vercel AI SDK v6 (I also renamed the old next-vercel-ai-sdk to next-vercel-ai-sdk-v5).
It uses @browser-ai that acts as a Transformers.js model provider for the Vercel AI SDK, enabling seamless tool-call management.

The internals of the provider handles incremental streaming that detects which tool is being called before the model finishes generating arguments, which is like how native cloud-based API's handle tool-call streaming.
It also automatically handles different model tool-call outputs, such as XML tags (<tool_call>) and Python-style ([functionName(args)]) syntax, which enhances the Developer Experience dramatically. Easily switch between Qwen3, LFM2, Granite models without introducing a bunch of boilerplate code.

On the UI side, it also allows for HITL (human in the loop) tool-calls by just setting a simple needsApproval flag in the tool-call schema.

@xenova let me know if anything needs to be changed :)

Demo:

demooo.mp4

[xenova]

Wow that's really cool! I'm sure @nico-martin will be interested too.

[jakobhoeg]

Thank you, @xenova!

Hopefully I can get the internals updated to use Transformers.js v4 too when it's released :)