[fix]: strip leading newline characters after 'pre' elements

[Tapped]

Close #7264

Strips the leading newline character after 'pre' elements per the spec https://www.w3.org/TR/2011/WD-html5-20110113/grouping-content.html#the-pre-element

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[Tapped]

I'm unsure if carriage return is already stripped from the source before feed into the parser?

Couldn't find any references to CRLF or '\r' in the parser, but I don't know.

[Tapped]

This change will cause issues for preserveWhitespace: true, as the leading newline after the pre tag is now not part of the HTML model (directly at least). We might want to consider refactoring how preserveWhitespace is done, as I think it's better if the parser eats leading and trailing whitespaces in contexts where it's allowed to, and stores the whitespace as a separate state on the HTML element, which can later in the HTML writer be used to preserve whitespaces. That will also likely remove almost identical whitespace handlers (trimmers) in sre_render and dom_render. Also I would expect preserveWhitespace to only be an option used in sre_render but seems to also be used in the dom_render for some reason.