Add OpenSearch description

[charmander]

Allows Weasyl search to be added to browsers.

• The icon is assets/img/favicon.svg. Its size as declared in opensearch.xml is 32x32 because current Firefox seems to render it at the declared size, then scale, even though it’s an SVG.

• The resource doesn’t have a hash suffix because it needs to self-update.

[codecov[bot]]

Codecov Report

Merging #1375 (5c4ad79) into main (a2298a4) will not change coverage. Report is 4 commits behind head on main. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1375   +/-   ##
=======================================
  Coverage   59.62%   59.62%           
=======================================
  Files          94       94           
  Lines        9069     9069           
  Branches     1636     1636           
=======================================
  Hits         5407     5407           
  Misses       3228     3228           
  Partials      434      434           

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