fix(removeUnknownsAndDefaults): keep href on the a element

[sjh9714]

Summary

Fixes #2230.

removeUnknownsAndDefaults deleted the href attribute from <a> elements, so
links in an optimized SVG stopped working:

optimize(`<svg viewBox="0 0 160 40" xmlns="http://www.w3.org/2000/svg">
  <a href="https://developer.mozilla.org/"><text x="10" y="25">MDN Web Docs</text></a>
</svg>`, { plugins: ['removeUnknownsAndDefaults'] });
// -> <a><text x="10" y="25">MDN Web Docs</text></a>

In _collections.js, elems.a pulls in the xlink attribute group — which is
why xlink:href survives — but its own attrs set never listed the SVG 2
href, so the plugin treated it as unknown. Twelve other entries in the same
file already list 'href' next to 'xlink:href' (image, use,
linearGradient, feImage, filter, mpath, script, …), so a looks like
an oversight.

Changes

• Add 'href' to elems.a.attrs in plugins/_collections.js (one line,
  alphabetically placed to match the surrounding entries)
• Add test/plugins/removeUnknownsAndDefaults.18.svg.txt covering both <a href> and <a xlink:href>; the xlink:href case is the control and passed
  before this change too

Testing

• yarn jest test/plugins/_index.test.js -t 'removeUnknownsAndDefaults'
• yarn test
• yarn lint
• yarn test:types

The new fixture fails on main (- <a href="https://developer.mozilla.org/"> /
+ <a>) and passes with the change. I did not run yarn test:regression or
yarn test:bundles locally.

Note on #2062

#2061/#2062 is the same class of bug for animation elements. That PR takes a
different route (a new animationTargetElement group) and does not touch
elems.a, so this is not a duplicate — but it also adds a
removeUnknownsAndDefaults.18.svg.txt. .18 is the next free index on main;
happy to renumber if #2062 lands first.