Expose generation timings from server & update completions.js (#2116)

* use javascript generators as much cleaner API

Also add ways to access completion as promise and EventSource

* export llama_timings as struct and expose them in server

* update readme, update baked includes

* llama : uniform variable names + struct init

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>