refactor: link libpros through pros-sys

[Tropix126]

Describe the changes this PR makes. Why should it be merged?

Removes the dependency on libpros.a in our target spec, instead opting to link through pros_sys only. I've kept libc for now as a target dependency, since we might want to eventually switch to the libc crate rather than forward declaring them in pros_sys.

Additional Context

We'll need to update the target spec in cargo-pros too.