Support concurrent translation

[vitonsky]

Currently we handle texts in one thread:

core/src/scheduling/Scheduler.ts

Line 433 in 3fcfd2a

const free = await this.semafor.take();

That make sense, because free services have strong rate limits.

However it works very bad with LLM translators. They are error prone, they generate a lot of errors and wait for 3-5 retries.
We could run at least 10 threads in parallel and show the translations faster.

Let's add the optional method like getConcurrency(): number and in case it is larger than 1 we will run translation in multiple threads.