[Post] Create pkg for post logic

[RaphaelPour]

Although a posts metadata has its own package, the post itself live within cmd/render and grows. While having everything within one cmd feels unstructured and semantically hard to read, it would be better to move the posts "business logic" to its own package. Better: rename the metadata package to post and extend it with the struct post from cmd/render. Maybe a second struct RenderedPost is needed.