Don't reset ::first-letter in Preflight

[adamwathan]

Resolves #13405

Previously we were applying our global reset rules to the ::first-letter pseudo to ensure it interacted with utility classes the same way as other elements, but touching that pseudo has some unfortunate side effects, including causing ::selection styles to not apply to the first letter as well as showing the default mouse cursor when hovering over the first letter instead of the text selection cursor.

I don't think there's anything really lost by removing this, because ::first-letter already has no padding or margin applied and doesn't support box-sizing. The only place it would have mattered previously is border styles, but now that we don't actually rely on Preflight in order to set border-style when adding a border-* utility, this will work fine anyways. Even if it didn't, it would be a better trade off to have to add first-letter:border-solid when necessary vs. dealing with the side-effects I've mentioned of touching that pseudo-element.