PostgreSQL: Support SQL standard ARRAY keyword in type declarations

[LucaCappelletti94]

Adds support for the SQL standard ARRAY keyword in array type declarations, such as INTEGER ARRAY and INTEGER ARRAY[4], which PostgreSQL accepts anywhere a type is expected (column definitions, casts, and the :: operator). It is gated behind a new Dialect::supports_array_typedef_with_keyword method, enabled for PostgreSQL, and the keyword form with its optional cardinality is represented by a new ArrayElemTypeDef::Keyword variant.

This also unifies the existing MySQL multi-valued index syntax CAST(... AS UNSIGNED ARRAY). The array: bool field on Expr::Cast is removed and the trailing ARRAY keyword is now captured through ArrayElemTypeDef::Keyword for every dialect, so this is a breaking change to the AST. Parser behavior is unchanged for all dialects, only the representation is unified, and round-trip serialization is preserved.