feat: Support generating function comments

[uki00a]

This PR adds support for generating function comments like the official Go plugin does.

• https://github.com/sqlc-dev/sqlc/blob/v1.27.0/examples/ondeck/postgresql/query/city.sql#L11-L21
• https://github.com/sqlc-dev/sqlc/blob/v1.27.0/examples/ondeck/postgresql/city.sql.go#L27-L35