[Request] __emitTokens to report relevance score

**Is your request related to a specific problem you're having?**
I'm developing a syntax definition using `__emitTokens` API, and this API doesn't allow me to report relevance, so my syntax couldn't be autodetected.

**The solution you'd prefer / feature you'd like to see added...**
The `emitter` object or some other argument passed to `__emitToken` should expose a function for reporting relevance.

**Any alternative solutions you considered...**
I studied HLJS implementation trying to find some way to directly reach the `relevance` variable, but didn't find such a way.

**Additional context...**
Also, it would be great to permit `__emitTokens` at `Mode` level. In such a scenario `__emitToken` could consume input up to certain point, where the current mode should end, and return the index of that point, so the core engine knew where to proceed.