stdlib: re-implement fs.watch as an iterator

[annieetang]

As part of the API audit/cleanup, we reimplement fs.watch as an iterator instead of passing in a callback so you can use fs.watch on a task cooperatively.

This will then have the issue that fs.walk has where for loops do not support yielding generalized iterators, so we can only implement with a while loop for now.

Usage could be something like

local watcher = fs.watch(tmpdir)

task.wait(0.1) -- Give some time for the watcher to start

fs.writestringtofile(watched, "x")

local foundChange = false
local startTime = os.clock()

local event = watcher:next()
while event and os.clock() - startTime < 2 do
	if event.change or event.rename then
		foundChange = true
	end
	event = watcher:next()
end

assert.eq(foundChange, true)
assert.eq(event == nil, true)

watcher:close()

See full examples in examples/watch_directory.luau or fs.test.luau:watch_iterator_on_change