Postgres regression 7b

[LucaCappelletti94]

• Add Expr::XmlConcat(Vec<Expr>) variant with Display and Spanned implementations for PostgreSQL's XMLCONCAT(xml[, ...]) function
• Introduce maybe_parse_xml_function dispatcher in the parser, hooked at the top of parse_function, which routes XML function names to dedicated AST nodes on dialects where supports_xml_expressions() is true
• Guards on a single unquoted identifier with case-insensitive matching, so only bare xmlconcat(...) on PostgreSQL and Generic dialects takes the new path — all other dialects and qualified/quoted names fall through to the normal function-call parser

This is the second PR in a chain splitting #2252 (XML function support) into reviewable units. Builds on #2299 which introduced the supports_xml_expressions() dialect method.