chore(docs): optionally set docs server port via VUE_STYLEGUIDIST_PORT

[ShGKme]

☑️ Resolves

• A bit personal request
• Allows running vue-styleguidist in main and stable8 in parallel locally

# main
npm run styleguide

# stable8
VUE_STYLEGUIDIST_PORT=6061 npm run styleguide

[ShGKme]

/backport to stable8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.58%. Comparing base (d21cce6) to head (496b8ea).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7805   +/-   ##
=======================================
  Coverage   51.58%   51.58%           
=======================================
  Files          96       96           
  Lines        3148     3148           
  Branches      866      866           
=======================================
  Hits         1624     1624           
  Misses       1276     1276           
  Partials      248      248           

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