PostgreSQL: Support more `COMMENT ON` variants

https://www.postgresql.org/docs/current/sql-comment.html specifies many different things that can receive database comments, but `sqlparser` currently only supports column, table, extension, schema, database, user and role. It would be great to add for example constraints, views and functions to the list too.

The issue names no source file or test. Start by locating the parser code and tests for existing COMMENT ON targets, then compare them with PostgreSQL's COMMENT ON documentation. Done means constraints, views, functions, and the other selected variants parse successfully without regressing the currently supported targets.

Written by the indexing model from the issue text.