Add "feed" aria role

[nolanlawson]

This role is defined in https://www.w3.org/TR/wai-aria-practices-1.1/#feed

[codecov-io]

Codecov Report

Merging #1124 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1124   +/-   ##
=======================================
  Coverage   91.48%   91.48%           
=======================================
  Files         125      125           
  Lines        4522     4522           
  Branches     1459     1459           
=======================================
  Hits         4137     4137           
  Misses        163      163           
  Partials      222      222

Impacted Files	Coverage Δ
src/validate/html/a11y.ts	91.86% <100%> (ø)	⬆️

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[Rich-Harris]

thank you!