rework luv work vm storage into userdata

[truemedian]

Moves work vm storage into a garbage collected userdata. Allowing for each lua state to manage its own storage. This completely removes the need for a global array of work vms.

Better than #755 because it does not introduce an extra interface that integrators have to deal with.

Essentially an expansion of #756 by using the userdata object to store the entire vm array.

In particular, this should mimic pre loop __gc behavior of multiple lua states having access to a pool of work vms and that pool being cleaned up automatically everywhere; the existing loop __gc does not handle this case well, particularly when one state is closed first (the other state may then attempt to use closed lua states).

Fixes #754

[clason]

That passes ASAN as well: https://github.com/neovim/neovim/actions/runs/13037209814/job/36370486555?pr=32174

[truemedian]

thread - getpriority, setpriority seems to be flaky with returning EINPROGRESS, not entirely sure why it's failing; libuv is however returning errno instead of the return value, so all we know is that it's failing (the errno value is not set by pthreads and is hence garbage).

[squeek502]

thread - getpriority, setpriority seems to be flaky with returning EINPROGRESS

#730

[Bilal2453]

I like this more than the previous globals approach frankly.

[zhaozg]

Looks great to me

[clason]

With this merged, luv now passes all Neovim tests: neovim/neovim#32174

So if you can make a new fixup release, I'll be able to include it. Thanks!