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.

[vitonsky]

I've tried to run the Linguist with LLM translator, it runs fine for single translations, but my UX of page translation is unacceptable bad.

So this feature must be prioritized.

Once this feature will be implemented, we could use concurrency:1 for a free translators, and concurrency:10 for LLM translators based on translator class config.

As result I expect significant improvement in page translation experience, since we will run translation for 10 batches at once, and once any batch will be resolved, we will start translation of another batch. It must hide bad latency that may reach 5 seconds for LLM models on OpenRouter.