[3.x] PoolVector to Vector

[lawnjelly]

Just an interesting diversion this morning, I remembered that we do have quite a few outstanding PoolVector bugs, and wondered how easy it would be to just make PoolVector a wrapper around Vector (as they both offer COW).

Some PoolVector bugs e.g.:
#69804
#51852
#53665
#37154

Turns out, for a basic version, it wasn't super tricky. So I'm making a draft here just for reference in future should be decide to go down that route.

There's probably quite a few flaws here as it's quick and dirty. In particular the writes are pretty inefficient because it will call the COW stuff each element is accessed (and the whole point of the Write is to avoid this), and we'd want them to reference the COW data to prevent de-allocation and dangling pointers (which the old version also suffers from). But it's a proof of concept.

Notes

• I did some basic benchmarking (fps in games, startup times for editor) and it's mostly around the same as vanilla Godot, maybe a little faster, but nothing to get worked up around.
• The main reason for making such a change would be to remove some of the PoolVector bugs, but otoh it is as likely that this would introduce regressions as much as solving existing bugs 😁 , which is why I'm just adding this as a draft rather than going full steam on it.
• 4.x notably just got rid of PoolVector altogether and replaced cases by Vector. That's probably something we could do in places in 3.x, but we'd need a PoolVector wrapper for backward compatibility I suspect, especially with bound languages.