bug: Add back case insensitivity to rule regex

[caitlinelfring]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Bug fix

What is the current behavior? (You can also link to an open issue here)
I accidentally removed (?i) in #100 so it no longer matches with case insensitivity

What is the new behavior (if this is a feature change)?
Add back case insensitivity

Does this PR introduce a breaking change? (What changes might users need to make due to this PR?)
No

Other information:
Fixes #108

[codecov]

Codecov Report

Merging #110 (0f9a235) into main (5b45f81) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #110   +/-   ##
=======================================
  Coverage   96.21%   96.22%           
=======================================
  Files          20       20           
  Lines         582      583    +1     
=======================================
+ Hits          560      561    +1     
  Misses         13       13           
  Partials        9        9           

Impacted Files	Coverage Δ
pkg/rule/rule.go	93.75% <100.00%> (+0.06%)	⬆️

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