project workflow improvements

[noamross]

• Use capsule for easier renv management
• Create a separate sandbox targets project
• Load multiple .env files
  • Use .env_user for setting USE_CAPSULE and TAR_PROJECT)
• .gitignore stuff appropriately to retain outputs and keep meta files
• Add all_targets() convenience function

I left off the s3 convenience functions because I realized they would work differently with regular targets and AWS "files", and wasn't yet sure how to handle it, but wanted to get these changes in first.