New seed policy

[wlandau]

Prework

• I understand and agree to the code of conduct and the contributing guidelines.
• I have already submitted a discussion topic or issue to discuss my idea with the maintainer.

Related GitHub issues and pull requests

• Ref: seed assignment based on target name has undesirable side effects #882

Summary

This PR implements user-side control of the seeds at the pipeline level. tar_option_set() now supports a seed argument, and target-specific seeds are determined by tar_option_get("seed") and the target name. tar_option_set(seed = NA) disables seed-setting behavior but forcibly invalidates all the affected targets except when seed is FALSE in the target's tar_cue().