Add option to configure the number of shake threads

[lukel97]

Only on ghcide and haskell-language-server

haskell/ghcide@3960533

[jneira]

Uhmm, this could be useful, but it seems to me that it will be a thing mainly oriented to ghcide/hls developers.
Not sure i it worths add a specific option visible to everybody, as you can make a wrapper for example and pass the argument there.

[lukel97]

True. We could alternatively just always pass -j 0, but I guess this depends on whether or not we want multithreading to be the default. Does anyone know if it’s considered experimental or stable?

[jneira]

Well, maybe a slightly more generic name could make it more user oriented: languageServerHaskell.numThreads or a boolean multithreading.
Is not -j 0 already the default value?

[lukel97]

You might be right. We don't pass -j 0 by default but I'll need to check if leaving out -j is actually multithreaded – I had just assumed it was the same behaviour as make

[lukel97]

After a bit of poking, it does look like ghcide sets the number of shake threads to 0 by default, which should mean it is by default multithreaded. But on haskell-language-server, it defaults to 1, so single threaded. I think this is probably more of an oversight in hls, I will address that there