Move to target lists

[wlandau]

Prework

• I understand and agree to the code of conduct and the contributing guidelines.
• I have already submitted an issue to the issue tracker to discuss my idea with the maintainer.
• This pull request is not a draft.

Related GitHub issues and pull requests

• Ref: Directly accept lists of target objects #253

Summary

This PR allows users to call list() instead of tar_pipeline(). _targets.R files should end with target lists, and those target lists can be arbitrarily nested (lists within lists).

# _targets.R
library(targets)
list(
  tar_target(data, get_data()),
  tar_target(analysis, analyze_data(data))
)

tar_pipeline() and tar_bind() still work but are deprecated with a warning.

FYI @limnoliver and @tjmahr

[codecov-io]

Codecov Report

Merging #254 (3188e70) into main (258f9e0) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main      #254      +/-   ##
===========================================
+ Coverage   99.85%   100.00%   +0.14%     
===========================================
  Files         136       136              
  Lines        4239      4270      +31     
===========================================
+ Hits         4233      4270      +37     
+ Misses          6         0       -6     

Impacted Files	Coverage Δ
R/tar_deduplicate.R	100.00% <ø> (ø)
R/tar_delete.R	100.00% <ø> (ø)
R/tar_destroy.R	100.00% <ø> (ø)
R/tar_glimpse.R	100.00% <ø> (ø)
R/tar_group.R	100.00% <ø> (ø)
R/tar_invalidate.R	100.00% <ø> (ø)
R/tar_load.R	100.00% <ø> (ø)
R/tar_make.R	100.00% <ø> (ø)
R/tar_make_clustermq.R	100.00% <ø> (ø)
R/tar_make_future.R	100.00% <ø> (ø)
... and 27 more

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