access dataset converters through CLI

[themattinthehatt]

Adds litpose convert <dataset_path> --lp_dir LP_DIR, replacing the standalone
scripts/converters/{dlc2lp,slp2lp}.py scripts. The command dispatches by input type: a directory is
treated as a DeepLabCut project, a .slp file as a SLEAP .pkg.slp package.

• New lightning_pose/converters/ package (dlc.py, sleap.py) with per-format convert() functions,
  ported from the old scripts with pathlib, logging, and type hints.
• Old scripts now exit with a pointer to the new CLI command.
• Docs (importing_labeled_data.rst, new cli_reference/convert.rst) updated accordingly.
• Tests added for both converters (synthetic DLC/.pkg.slp fixtures) and the CLI dispatch logic.

[codecov]

Codecov Report

❌ Patch coverage is 93.95604% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.97%. Comparing base (7182845) to head (96a0a7b).

Files with missing lines	Patch %	Lines
lightning_pose/converters/sleap.py	93.81%	6 Missing ⚠️
lightning_pose/converters/dlc.py	91.66%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #490      +/-   ##
==========================================
+ Coverage   92.94%   92.97%   +0.02%     
==========================================
  Files          62       65       +3     
  Lines        5968     6150     +182     
==========================================
+ Hits         5547     5718     +171     
- Misses        421      432      +11     

Files with missing lines	Coverage Δ
lightning_pose/cli/commands/__init__.py	100.00% <ø> (ø)
lightning_pose/cli/commands/convert.py	100.00% <100.00%> (ø)
lightning_pose/data/utils.py	98.88% <ø> (ø)
lightning_pose/converters/dlc.py	91.66% <91.66%> (ø)
lightning_pose/converters/sleap.py	93.81% <93.81%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.