Rename action levels

[yjunechoe]

Summary

A TODO list for renaming action levels:

User-facing functions:

• Implement action_levels() soft deprecation safeguards
• Add new params to action_levels() documentation
• Same deprecation treatment to warn_on_fail()
• Add error_on_fail(error) to deprecate stop_on_fail(stop_at)
• Deprecation handling for fns (now fns = action_fns(warn, error, critical) Formalizing a <action_fns> object #612 )
• Export and document action_fns() (perhaps this PR can implement this internally and exporting can be separate PR)
• get_agent_x_list() rename $stop, $notify

Display/write:

• <action_levels> prints using new names
• {cli} messages use STOP->ERROR and NOTIFY->CRITICAL
• get_agent_report() (report_tbl)
• get_multiagent_report()
• Draft validation should use new names

YAML:

• Write using new names
• Reading should handle BOTH old and new variants

Internals (low priority):

• Rename innards of <action_levels> object (ex: $stop_fraction -> $error_fraction)
• Renaming in agent$validation_set (related to get_agent_x_list() output)

Documentation:

• Deprecation badges for old params
• Re-write description of action levels in action_levels() documentation
• Re-generate images of reports showing W/E/C
• add NEWS

Find-and-replace:

• Internal uses of action_levels() should use new names (ex: in "::QUIET::" agents for test/expect functions)
• Change roxygen examples to use new names
• Use new names in all of /tests

Tests:

• Write tests for deprecation signals/safeguards

Related GitHub Issues and PRs

• Ref: overhaul action levels to log levels #604

Checklist

• I understand and agree to the Code of Conduct.
• I have listed any major changes in the NEWS.
• I have added testthat unit tests to tests/testthat for any new functionality.

[yjunechoe]

Still working my way through this but just had a thought:

Since the diffs are getting huge (due to find-and-replace), I think I'll revert those before I open this PR for review (which should only generate deprecation warnings in the checks), and then I can re-apply the name changes (which should resolve those deprecation warnings and put us back on track -- this can also be done in a separate PR to chunk things up).