Move {rstudioapi} to `Suggets:`

[lorenzwalthert]

I suggest to make {rstudioapi} a suggested dependency as it's not required for many workflows. I am currently looking into reducing the dependency graph of {precommit} dependencies as outlined in lorenzwalthert/precommit#356, which is why I file this issue. It seems as {rstudioapi} is only used once in the source code of {pkgload}.