feat(turborepo): Persistent Tasks in Watch Mode (#7922)
### Description
Implements handling of persistent tasks. Basically, we strip out any
persistent tasks from the task graph when we do either an execution for
package rediscovery or package changes. Instead, we run the persistent
tasks in a separate thread, so they don't block anything.
### Testing Instructions
Added tests for both pruning methods.
Closes TURBO-2775
