targets 0.4.0

Bug fixes

Avoid file.rename() errors when migrating staged temporary files (#410).
Return correct error messages from feather and parquet formats (#388). Now calling assert_df() from store_assert_format() instead of store_cast_object(). And now those last two functions are not called at all if the target throws an error.
Retry writing lines to database files so Windows machines can run tar_poll() at the same time as the pipeline (#393).
Rename file written by tar_renv() to _targets_packages.R (#397).
Ensure metadata is loaded to compute labels properly when outdated = FALSE in tar_visnetwork().

Implement tar_timestamp() and tar_timestamp_raw() to get the last modified timestamp of a target's data (#378).
Implement tar_progress_summary() to compactly summarize all pipeline progress (#380).
Add a characters argument of tar_traceback() to cap the traceback line lengths (#383).
Add new "summary" and "about" views to tar_watch() (#382).
Implement tar_poll() to repeatedly poll runtime progress in the R console (#381). tar_poll() is a lightweight alternative to tar_watch().
Change the color of the "dormant" status in the graph.
Add a tar_envvar() function to list values of special environment variables supported in targets. The help file explains each environment variable in detail.
Support extra project-level configuration settings with _targets.yaml (#297). New functions tar_config_get() and tar_config_set() interact with the _targets.yaml file. Currently only supports the store field to set the data store path to something other than _targets/.

Shut down superfluous persistent workers earlier in dynamic branching and when all remaining targets have deployment = "main" (#398, #399, #404, @pat-s).

Attempt to print only the useful part of the traceback in tar_traceback() (#383).
Add a line break at the end of the "summary" reporter so warnings do not mangle the output.
In tar_watch(), use shinybusy instead of shinycssloaders and keep current output on display while new output is rendering (#386, @rcorty).
Right-align the headers and counts in the "summary" and "forecast" reporters.
Add a timestamp to the "summary" reporter.
Make the reporters show when a target ends (#391, @mattwarkentin).
Make the reporters show when a pattern ends if the pattern built at least one target and none of the targets errored or canceled.
Use words "start" and "built" in reporters.
Use the region of the AWS S3 bucket instead of the local AWS_DEFAULT_REGION environment variable (check_region = TRUE; #400, @tomsing1).
In tar_meta(), return POSIXct times in the time zone of the calling system (#131).
Throw informative error messages when a target's name or command is missing (#413, @liutiming).
Bring back ALTREP in qs::qread() now that qs 0.24.1 requires stringfish >= 1.5.0 (#147, @glep).
Relax dynamic branching checks so pattern = slice(...) can take multiple indexes (#406, #419, @djbirke, @alexgphayes)