Core themes add fetchpriority=high to the site logo, to improve page rendering performance

Adding a directive to the browser that an image above the fold needs to be downloaded helps with overall site performance.

Previously core themes had:

<img src="{{ site_logo }}" alt="{{ 'Home'|t }}" />

Now core themes have:

<img src="{{ site_logo }}" alt="{{ 'Home'|t }}" fetchpriority="high" />