Implement new MFM syntax

src/mfm/parse/elements/big.ts

@@ -0,0 +1,20 @@
+/**
+ * Bold
+ */
+
+export type TextElementBig = {
+	type: 'big'
+	content: string
+	big: string
+};
+
+export default function(text: string) {
+	const match = text.match(/^\*\*\*(.+?)\*\*\*/);
+	if (!match) return null;
+	const big = match[0];
+	return {
+		type: 'big',
+		content: big,
+		big: match[1]
+	} as TextElementBig;
+}