[MFM] Add spin syntax

Resolve #4003

src/mfm/html.ts

@@ -55,6 +55,12 @@ export default (tokens: MfmForest, mentionedRemoteUsers: INote['mentionedRemoteU
 			return el;
 		},
 
+		spin(token) {
+			const el = doc.createElement('i');
+			appendChildren(token.children, el);
+			return el;
+		},
+
 		blockCode(token) {
 			const pre = doc.createElement('pre');
 			const inner = doc.createElement('code');