feat: allow hilbish exits to be interrupted

[sammy-ette]

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