Move the Block helper type to its own file

Collapse the various standalone functions for parsing block data, and
refactor consumers to initialize a Block with given data and access its
members as needed.

Closes: https://github.com/damus-io/damus/pull/1496
Signed-off-by: William Casarin <jb55@jb55.com>

damus/Views/DMChatView.swift

@@ -130,7 +130,9 @@ struct DMChatView: View, KeyboardReadable {
     func send_message() {
         let tags = [["p", pubkey.hex()]]
         let post_blocks = parse_post_blocks(content: dms.draft)
-        let content = render_blocks(blocks: post_blocks)
+        let content = post_blocks
+            .map(\.asString)
+            .joined(separator: "")
 
         guard let dm = create_dm(content, to_pk: pubkey, tags: tags, keypair: damus_state.keypair) else {
             print("error creating dm")

damus/Views/NoteContentView.swift

@@ -445,7 +445,15 @@ func render_blocks(blocks bs: Blocks, profiles: Profiles) -> NoteArtifactsSepara
     let blocks = bs.blocks
     
     let one_note_ref = blocks
-        .filter({ $0.is_note_mention })
+        .filter({
+            if case .mention(let mention) = $0,
+               case .note = mention.ref {
+                return true
+            }
+            else {
+                return false
+            }
+        })
         .count == 1
     
     var ind: Int = -1