Fix multiple runtime APIs that were incorrectly using std::string::reserve

[vrn-sn]

Problem

Multiple runtime APIs incorrectly were using std::string::reserve where they should have been using std::string::resize. This led to UB that seemed to behave fine at runtime, but the introduction of #467 caused all of these APIs to break.

Solution

We introduce uvutils::getStringFromUv as a utility API to help reduce errors when interacting with libuv. Internally, this handles all the buffer logic and simply returns a value or an error to the caller.

In fixing this, I also discovered two issues with our CI:

1. One of the checks in path.posix.test.luau is flipped but was mistakenly passing because of the previously broken process.cwd() API.
2. Our Luau tests don't even run on Windows! 😞

I've fixed (1) and have made a separate issue for (2): #483.

[Vighnesh-V]

Requesting some small changes, but this broadly looks good to go.

[vrn-sn]

Currently also trying to figure out why Windows tests are failing now that I'm trying to re-enable them.

Edit: giving up on this for now and reverting these Windows-related changes.

[Vighnesh-V]

Some style comments, but not blocking.