make useLink concurrent safe

[JoviDeCroock]

Resolve #69

Rather than always looking for an existing link we'll assume that when we come from hydration we'll see data-hoofd: "1" which we'll then take over and hydrate with. Not entirely sure whether this is a breaking change yet 😅 it does solve our concurrent mode problems

Example: https://codesandbox.io/s/priceless-villani-lletf8?file=/src/App.js:174-239

[github-actions]

Size Change: +392 B (4%)

Total Size: 9.24 kB

Filename	Size	Change
dist/hoofd.js	2.3 kB	+97 B (4%)
dist/hoofd.modern.js	2.27 kB	+105 B (4%)
dist/hoofd.module.js	2.32 kB	+97 B (4%)
dist/hoofd.umd.js	2.35 kB	+93 B (3%)

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