ci: add publiccode.yml validation workflow

[publiccode-pr-bot]

This PR adds a GitHub Actions workflow that validates publiccode.yml on every pull request using italia/publiccode-parser-action@v1.

No behavior change to the code, just CI.

[codecov]

Bundle Report

Bundle size has no change ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.17%. Comparing base (0fe0550) to head (355cd5a).
⚠️ Report is 19 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #499   +/-   ##
=======================================
  Coverage   92.17%   92.17%           
=======================================
  Files          69       69           
  Lines        2287     2287           
  Branches      582      575    -7     
=======================================
  Hits         2108     2108           
- Misses        160      173   +13     
+ Partials       19        6   -13     

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[brunobuddy]

The validator added does not accept SVG and GIFs as images... This PR is not bringing value to the project