[MFM] Add center syntax

Resolve #1775

src/mfm/parser.ts

@@ -73,6 +73,7 @@ const mfm = P.createLanguage({
 		r.math,
 		r.search,
 		r.title,
+		r.center,
 		r.text
 	).atLeast(1),
 
@@ -113,6 +114,23 @@ const mfm = P.createLanguage({
 		).atLeast(1).tryParse(x))),
 	//#endregion
 
+	//#region Center
+	center: r =>
+		P.regexp(/<center>([\s\S]+?)<\/center>/, 1)
+		.map(x => makeNodeWithChildren('center', P.alt(
+			r.big,
+			r.bold,
+			r.motion,
+			r.mention,
+			r.hashtag,
+			r.emoji,
+			r.math,
+			r.url,
+			r.link,
+			r.text
+		).atLeast(1).tryParse(x))),
+	//#endregion
+
 	//#region Emoji
 	emoji: r =>
 		P.alt(