WIP: expression support

[rabernat]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 87.01299% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.38%. Comparing base (df782a2) to head (439e37a).

Files with missing lines	Patch %	Lines
src/xpublish_tiles/pipeline.py	82.60%	8 Missing ⚠️
src/xpublish_tiles/expressions.py	96.29%	1 Missing ⚠️
src/xpublish_tiles/xpublish/tiles/plugin.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
- Coverage   88.40%   88.38%   -0.02%     
==========================================
  Files          22       23       +1     
  Lines        2742     2815      +73     
==========================================
+ Hits         2424     2488      +64     
- Misses        318      327       +9     

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