Add support for `<?xml` declarations

[charlespwd]

Unformatted source

<?xml version="1.0" encoding="UTF-8"?>

Expected output

<?xml version="1.0" encoding="UTF-8"?>

Actual output

LiquidHTMLParsingError

[charlespwd]

According to MDN, Processing instructions are not valid in HTML documents (they actually declare that the document is an XML document).

I'm lowering the priority of this as I let my backburner decide what to do with this (e.g. do we switch parser? Do we not care? Do we throw an error? Do we let it as is?)

It seems like enough of an edge case to ignore.