Extend RNG seed functionality

targets 0.14.0

• Move #!/bin/sh line to the top of SLURM clustermq template file (#944, #955, @GiuseppeTT).
• Add new function tar_path_script().
• Rename tar_store() to tar_path_store() with deprecation.
• Rename tar_path() to tar_path_target() with deprecation.
• Add new function tar_path_script_support().
• Make Target Markdown target scripts dynamically locate their support scripts so the appropriate scripts can be found even when they are generated from one directory and sourced from another (#953, #957, @TylerGrantSmith).
• Allow 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() (#882, @sworland-thyme, @joelnitta).
• Implement a seed argument in tar_cue() to control whether targets update in response to changing or NA seeds (#882, @sworland-thyme, @joelnitta).
• Reduce the number of per-target AWS/GCP storage API calls. Previously there were 3 API calls per target, including 2 HEAD requests. Now there is just 1 for a typical target (unless dependencies have to be downloaded). Relies on S3 strong read-after-write consistency (#958).
• Update the tar_github_actions() workflow file to use @v2 (#960, @kulinar).
• Print helpful hints while debugging a target interactively (#961).
• Only attempt to debug a target when callr_function is NULL (#961).
• Make formats "feather", "parquet", "file", and "url" work with error = "null" (#969).
• Declare formats "keras" and "torch" superseded by tar_format(). Documented in the tar_target() help file.
• Declare formats "keras" and "torch" incompatible with error = "null". Documented in the tar_target() help file and in a warning thrown by tar_target() via tar_target_raw().
• Add a convert argument to tar_format() to allow custom store_convert_object() methods (#970).