Reject nested values as query boundary

[schmidt-sebastian]

This adds validation that a query can only start or end at a document that is a direct child of the query's collection.

[codecov]

Codecov Report

Merging #60 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #60   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          11     11           
  Lines        1479   1481    +2     
=====================================
+ Hits         1479   1481    +2

Impacted Files	Coverage Δ
src/reference.js	100% <100%> (ø)	⬆️

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