Add JSON.document_root; test JSON and JSONSchema

[handrews]

Implements part of https://github.com/marksparkza/jschon/discussions/108#discussioncomment-6471856 This adds a generic document_root cached property to the JSON class, which can be distinct from both the document_schemaroot and resource_schemaroot of the JSONSchema class.

This lays groundwork to support mixed content such as OpenAPI.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% :tada:

Comparison is base (e6f71d0) 92.73% compared to head (93b9ddb) 92.74%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
+ Coverage   92.73%   92.74%   +0.01%     
==========================================
  Files          23       23              
  Lines        2052     2055       +3     
  Branches      435      436       +1     
==========================================
+ Hits         1903     1906       +3     
  Misses         97       97              
  Partials       52       52              

Files Changed	Coverage Δ
jschon/json.py	94.28% <100.00%> (+0.08%)	:arrow_up:

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