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.

該 issue 未指定原始檔案或測試。先找出現有 COMMENT ON 目標的 parser 程式碼與測試，然後將它們與 PostgreSQL 的 COMMENT ON 文件進行比較。完成的標準是 constraints、views、functions 以及其他選定的變體都能成功解析，同時不會讓目前支援的目標發生回歸。

由索引模型根據 Issue 內容生成。