fix: diagnostic error for interface arguments

[gnikit]

This commit fixes a bug where the sub children of an
interface would not be properly checked for their letter case.

Fixes #471

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.33%. Comparing base (f7c3ecd) to head (a7a2199).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #472   +/-   ##
=======================================
  Coverage   88.33%   88.33%           
=======================================
  Files          35       35           
  Lines        4800     4800           
=======================================
  Hits         4240     4240           
  Misses        560      560           

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