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.

该 issue 未指定源文件或测试。首先定位现有 COMMENT ON 目标对应的 parser 代码和测试，然后将其与 PostgreSQL 的 COMMENT ON 文档进行比较。完成的标准是 constraints、views、functions 及其他选定变体都能成功解析，同时不会导致当前支持的目标出现回归。

由索引模型根据 Issue 内容生成。