[MFM] Add spin syntax

Resolve #4003

src/client/app/animation.styl

@@ -26,3 +26,8 @@
 		transform: translateY(0);
 	}
 }
+
+@keyframes spin {
+	0% { transform: rotate(0deg); }
+	100% { transform: rotate(360deg); }
+}

src/client/app/common/views/components/mfm.ts

@@ -124,6 +124,17 @@ export default Vue.component('misskey-flavored-markdown', {
 					}, genEl(token.children));
 				}
 
+				case 'spin': {
+					motionCount++;
+					const isLong = sumTextsLength(token.children) > 5 || countNodesF(token.children) > 3;
+					const isMany = motionCount > 3;
+					return (createElement as any)('span', {
+						attrs: {
+							style: (this.$store.state.settings.disableAnimatedMfm || isLong || isMany) ? 'display: inline-block;' : 'display: inline-block; animation: spin 1.5s linear infinite;'
+						},
+					}, genEl(token.children));
+				}
+
 				case 'url': {
 					return [createElement(MkUrl, {
 						key: Math.random(),

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');

src/mfm/parser.ts

@@ -91,6 +91,7 @@ const mfm = P.createLanguage({
 	root: r => P.alt(
 		r.big,
 		r.small,
+		r.spin,
 		r.bold,
 		r.strike,
 		r.italic,
@@ -122,6 +123,7 @@ const mfm = P.createLanguage({
 			r.hashtag,
 			r.emoji,
 			r.math,
+			r.spin,
 			r.text
 		).atLeast(1).tryParse(x), {})),
 	//#endregion
@@ -140,6 +142,15 @@ const mfm = P.createLanguage({
 		).atLeast(1).tryParse(x), {})),
 	//#endregion
 
+	//#region Spin
+	spin: r =>
+		P.regexp(/<spin>(.+?)<\/spin>/, 1)
+		.map(x => createTree('spin', P.alt(
+			r.emoji,
+			r.text
+		).atLeast(1).tryParse(x), {})),
+	//#endregion
+
 	//#region Block code
 	blockCode: r =>
 		newline.then(
@@ -173,6 +184,7 @@ const mfm = P.createLanguage({
 		.map(x => createTree('center', P.alt(
 			r.big,
 			r.small,
+			r.spin,
 			r.bold,
 			r.strike,
 			r.italic,
@@ -261,6 +273,7 @@ const mfm = P.createLanguage({
 			return createTree('link', P.alt(
 				r.big,
 				r.small,
+				r.spin,
 				r.bold,
 				r.strike,
 				r.italic,
@@ -304,6 +317,7 @@ const mfm = P.createLanguage({
 		.map(x => createTree('motion', P.alt(
 			r.bold,
 			r.small,
+			r.spin,
 			r.strike,
 			r.italic,
 			r.mention,
@@ -364,6 +378,7 @@ const mfm = P.createLanguage({
 			const contents = P.alt(
 				r.big,
 				r.small,
+				r.spin,
 				r.bold,
 				r.strike,
 				r.italic,