Add initial pipeline diagramming support and some CLI refactors / cleanups

[mdekstrand]

This adds support for rendering Mermaid diagrams of LensKit pipelines, along with some CLI refactor and test improvements now that we're loading pipelines in more different commands. Closes #1038.

[codecov]

Codecov Report

❌ Patch coverage is 86.50000% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.67%. Comparing base (2c532cd) to head (4a7df3a).

Files with missing lines	Patch %	Lines
src/lenskit/cli/__init__.py	47.36%	10 Missing ⚠️
src/lenskit/cli/pipeline/diagram.py	73.68%	10 Missing ⚠️
src/lenskit/pipeline/_diagram.py	92.06%	5 Missing ⚠️
src/lenskit/cli/pipeline/_load.py	95.12%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1100      +/-   ##
==========================================
+ Coverage   89.65%   89.67%   +0.01%     
==========================================
  Files         243      247       +4     
  Lines       16575    16729     +154     
==========================================
+ Hits        14861    15002     +141     
- Misses       1714     1727      +13     

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