[EPIC] Get target-size ready to be enabled by default

With WCAG 2.2 increasingly becoming the standard of choice (44% in Axe extension today), and EN 301 549 v4 switching over to use it we need to firm up the `target-size` rule. When we built this we released it knowing there were limitations, but figuring because it was off by default it'd be okay. That is changing, so the rule needs to be improved.