Isolate cache per doc loader and add cache instance option.

[dlongley]

No description provided.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (00bcc62) to head (78b6a93).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #11   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           43        44    +1     
=========================================
+ Hits            43        44    +1     

Files with missing lines	Coverage Δ
lib/security-loader.js	100.00% <100.00%> (ø)

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