Change italic syntax to fit with other markdown implementations (#3916)

currently italics require an html <i> syntax, unlike every other formatter. this is very confusing, especially as there doesnt seem to be any documentation.
this change makes both _text_ and *text* work, both things that users would expect from other implementations of markdown such as pleroma, discord and github.

Add test

src/mfm/parser.ts

@@ -224,7 +224,7 @@ const mfm = P.createLanguage({
 
 	//#region Italic
 	italic: r =>
-		P.regexp(/<i>([\s\S]+?)<\/i>/, 1)
+		P.regexp(/(\*|_)([a-zA-Z0-9]+?[\s\S]*?)\1/, 2)
 		.map(x => createTree('italic', P.alt(
 			r.bold,
 			r.strike,