Add 'current' to the list of aria attributes

[EmilTholin]

This PR adds current to the list of aria attributes, so that Svelte doesn't give a A11y: Unknown aria attribute 'aria-current' warning when aria-current is used.

[codecov-io]

Codecov Report

Merging #953 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #953   +/-   ##
=======================================
  Coverage   91.74%   91.74%           
=======================================
  Files         109      109           
  Lines        4010     4010           
  Branches     1287     1287           
=======================================
  Hits         3679     3679           
  Misses        148      148           
  Partials      183      183

Impacted Files	Coverage Δ
src/validate/html/a11y.ts	94.93% <100%> (ø)	⬆️

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[Rich-Harris]

Thank you!