feat(module:avatar): add loading and fetchpriority attributes

[arturovt]

Closes #7274

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.49%. Comparing base (512a577) to head (f43d6f2).
Report is 33 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7347   +/-   ##
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  Coverage   91.49%   91.49%           
=======================================
  Files         534      534           
  Lines       18425    18425           
  Branches     2815     2815           
=======================================
  Hits        16858    16858           
  Misses       1246     1246           
  Partials      321      321           

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[simplejason]

As caniuse->loading and caniuse->fetchpriority shown, these attributes have low compatibility for most browsers, I don't think we need built-in properties to support. What do you think about this @hsuanxyz ?