Custom descriptions

targets 1.6.0

• Modernize extras in tar_renv().
• tar_target() gains a description argument for free-form text describing what the target is about (#1230, #1235, #1236, @tjmahr).
• tar_visnetwork(), tar_glimpse(), tar_network(), tar_mermaid(), and tar_manifest() now optionally show target descriptions (#1230, #1235, #1236, @tjmahr).
• tar_described_as() is a new wrapper around tidyselect::any_of() to select specific subsets of targets based on the description rather than the name (#1136, #1196, @noamross, @mattmoo).
• Fix the documentation of the names argument (nudge users toward tidyselect expressions).
• Make assertions on the pipeline process more robust (to check if two processes are trying to access the same data store).