add neq as a supported parameter

[philvarner]

Related Issue(s): #214

Description:

• support neq operator in text format

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to the CHANGELOG

[codecov-commenter]

Codecov Report

Merging #216 (df93184) into main (055e2b2) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #216      +/-   ##
==========================================
+ Coverage   83.75%   83.79%   +0.04%     
==========================================
  Files           9        9              
  Lines         677      679       +2     
==========================================
+ Hits          567      569       +2     
  Misses        110      110              

Impacted Files	Coverage Δ
pystac_client/cli.py	62.18% <100.00%> (+0.32%)	⬆️
pystac_client/item_search.py	91.98% <100.00%> (+0.02%)	⬆️

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