Add rules for checking package index filename

[per1234]

The package index filename must follow a specific format. Package indexes which are not compliant are ignored by the
Arduino development software.

Only the official package index may be named package_index.json.

[codecov-commenter]

Codecov Report

Merging #184 (b24f9e2) into main (53c1162) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
+ Coverage   84.45%   84.50%   +0.05%     
==========================================
  Files          43       43              
  Lines        3017     3027      +10     
==========================================
+ Hits         2548     2558      +10     
  Misses        375      375              
  Partials       94       94              

Flag	Coverage Δ
unit	84.50% <100.00%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ternal/rule/ruleconfiguration/ruleconfiguration.go	100.00% <ø> (ø)
internal/rule/rulefunction/packageindex.go	82.60% <100.00%> (+13.37%)	⬆️

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