Forbid short document keys like $k

[AlekSi]

Closes #181.

[codecov]

Codecov Report

Merging #234 (c39fb58) into main (9ccdcd4) will increase coverage by 0.57%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #234      +/-   ##
==========================================
+ Coverage   47.69%   48.26%   +0.57%     
==========================================
  Files         107      107              
  Lines        4982     4985       +3     
==========================================
+ Hits         2376     2406      +30     
+ Misses       2182     2164      -18     
+ Partials      424      415       -9     

Impacted Files	Coverage Δ
internal/types/document.go	62.40% <100.00%> (+5.48%)	⬆️
internal/bson/document.go	43.36% <0.00%> (+2.91%)	⬆️
internal/wire/op_msg.go	45.85% <0.00%> (+2.92%)	⬆️
internal/wire/msg_body.go	36.50% <0.00%> (+4.76%)	⬆️
internal/bson/array.go	53.33% <0.00%> (+5.00%)	⬆️

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