feat: add process.execpath to get the current lute executable

[Nerixyz]

I was trying to get the code generation to run through CMake (which would run lute in the build). When integrating this into luthier, I needed a way to identify the current lute executable to pass to CMake.

This adds execpath to @lute/process, which mirrors Node.js' process.execPath to get the lute executable path - this is immune to exec -a foo lute having foo as argv[0].

uv_setup_args can allocate on some platforms. That memory should be free'd with uv_library_shutdown. I'm not sure where it's appropriate to call this.

Passing around the argv0 also seems a bit wrong. Maybe this path shouldn't be stored in Runtime?

[Nerixyz]

Looks like uv wants uv_library_shutdown to be called once per "process", because it uses a local static: https://github.com/libuv/libuv/blob/a944c422cca5522073e03710ca7fd08f53218358/src/uv-common.c#L975-L978. So in the tests, the library is shut down only once.
Not sure what's the best way to resolve this. Maybe uv's logic for getting the executable name should be implemented here?