threads.js has better support for modules - no need to give a
project-relative path to the worker file, which complicated the build.
Add rudimentary thread pooling w/ execution timeout.
Moves sudoku generation off of the main thread. Allows for multiple
generation requests (up to the number of physical CPU cores) to be
served in parallel. Uses the "physical-cpu-count" node package to
determine the numer of physical CPUs of the host.
Also introduces a timeout which causes too-difficult (or impossible)
generation requests to fail if they take more than 5 seconds to
complete. In effect soft-capping the complexity of generated puzzles.
