Rename "Upload an asset" slash command

Given Logseq’s transition into a desktop app (and its local nature), I think that the word “upload” no longer makes sense.

It may also give new users the impression that their asset is going to be uploaded to some cloud storage provider, when in fact it will just be copied to the assets folder.

Also, the word “asset” feels a bit too technical, so if an alternative term is possible, I would suggest “Insert file”, “Add media” or something to that effect.