Set up Psalm

[gsteel]

Initial setup for psalm, baselining existing issues.

Updates .gitattributes to export-ignore relevant tool config

Closes #159

[codecov]

Codecov Report

Base: 90.85% // Head: 90.85% // No change to project coverage 👍

Coverage data is based on head (625f3ec) compared to base (ffaa564).
Patch has no changes to coverable lines.

❗ Current head 625f3ec differs from pull request most recent head b49489c. Consider uploading reports for the commit b49489c to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##              1.2.x     #160   +/-   ##
=========================================
  Coverage     90.85%   90.85%           
  Complexity      128      128           
=========================================
  Files            25       25           
  Lines           317      317           
=========================================
  Hits            288      288           
  Misses           29       29           

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