test(inflection): add test cases for 'regimen' pluralization

[theothersideofgod]

Problem:

• The 'regimen' word requires special handling for correct pluralization
• Need to ensure that inflection functions (plural/singular) correctly handle 'regimen' -> 'regimens' and 'regimens' -> 'regimen' transformations

Solution:

• Added test cases for 'regimen' and 'regimens' in both plural() and singular() test suites
• These tests verify that:
  • plural('regimen') returns 'regimens'
  • plural('regimens') returns 'regimens' (already plural)
  • singular('regimens') returns 'regimen'
  • singular('regimen') returns 'regimen' (already singular)

Changes:

• Added 'regimen' and 'regimens' test cases to plural() test suite
• Added 'regimens' and 'regimen' test cases to singular() test suite

Testing:

• Tests verify correct pluralization behavior for 'regimen' word
• Ensures consistency with other special cases (children, men, etc.)