fix: keep pseudo-element rules that can't be inlined

[cossssmin]

Closes #293

With the default removeStyleTags: true, rules using ::first-letter, ::first-line, ::marker, ::selection or ::placeholder were silently dropped: they can't be inlined, but they weren't kept in the <style> tag either. So a drop cap like this one disappeared completely, even in email clients that support ::first-letter:

<style>h3::first-letter { color: red; font-size: 32px; }</style>
<h3>abc</h3>

These now go in the default ignoredPseudos, next to hover, active and friends, which is exactly what that list is for: skip inlining, keep the rule in a <style> tag (controlled by preservePseudos). Both :: and the legacy : syntax work.

Output for the example above:

<style>
h3::first-letter {
  color: red;
  font-size: 32px;
}
</style><h3>abc</h3>

Note that this keeps ::first-letter rules in the <style> tag rather than inlining them as <span>s like inlinePseudoElements does for ::before / ::after. Wrapping the first letter correctly (leading punctuation, emoji, nested elements, interplay with ::before) is a lot more involved, and ::first-line can't be done statically at all.

Added tests for each pseudo-element, both syntaxes, preservePseudos: false, and regular rules still being inlined alongside. Also updated the ignoredPseudos default in the README.

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