Add an API to boost a task’s priority

[ahoppen]

There should be an API to increase a task’s priority. I am currently working around the lack of such an API using

Task(priority: targetPriority) {
  await self.resultTask.value
}

Konrad mentioned that we should offer __priorityBoost(task, priority) here: apple/sourcekit-lsp#1208 (comment)

[ktoso]

Thanks for the issue, yes I think this is something we likely should offer even if for spi use for now, because we may have situations where due to other information we know to boost a task -- kind of like your scheduler does.