PostgreSQL supports XMLPARSE(CONTENT expr) and XMLPARSE(DOCUMENT expr), which cannot be represented as a plain function call because of the leading CONTENT or DOCUMENT mode keyword. This follows how the parser already models other keyword-syntax functions such as EXTRACT(field FROM expr), adding an XmlParse expression with an XmlParseMode of CONTENT or DOCUMENT. It is parsed for dialects that return true from supports_xml_expressions(), namely PostgreSQL and Generic, and on every other dialect xmlparse keeps parsing as an ordinary function.

Part of the work to support PostgreSQL XML functions (split from #2252, follows #2299 and #2368).