feat: add string entropy greater than filter

[didroe]

Description

Adds a new filter type entropy_greater_than which takes a decimal number and compares it to the Shannon Entropy of the string.

This will be used to improve the PHP hardcoded secret rule which currently triggers many false positives.

Checklist

• I've added test coverage that shows my fix or feature works as expected.
• I've updated or added documentation if required.
• I've included usage information in the description if CLI behavior was updated or added.
• PR title follows Conventional Commits format