feat: allow hilbish exits to be interrupted

[TorchedSammy]

if background jobs are being run, the user should be reminded and stopped from exiting the shell. requires #297

simple implementation code:

function hilbish.exit()
    local lingerers = bait.throw 'hilbish.exit'
    if #lingerers == 0 then
        os.exit(0)
    end

    for _, linger in pairs(lingerers)
        print('Woah! Can\'t leave this flower field yet. Reason: %s', linger.reason)
    end
end