fix(styling): properly import Vanilla-Calendar CSS and only once

[ghiscoding]

• we should have only 1 import and for it to be properly imported in SASS, we need to remove the file externsion when importing
• fixes invalid import and missing CSS when tested in Salesforce

[stackblitz]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.7%. Comparing base (240cbd3) to head (4c99671).

Additional details and impacted files

@@          Coverage Diff          @@
##            next   #1492   +/-   ##
=====================================
  Coverage   99.7%   99.7%           
=====================================
  Files        197     197           
  Lines      21652   21652           
  Branches    6938    6938           
=====================================
  Hits       21586   21586           
  Misses        66      66           

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