As far as I know there is nothing like that. But I also don't see why anyone would need a feature like that, because there is no way in Unity to put text from a file directly into a UI text element. There is always at least one step in between where you read the content of the text file into a string variable. And this is where I would approach this problem.
For example, you could say that when a line in your text file begins with {
(or some other character which is unlikely to show up in regular dialogue text) then this line is the facial expression. When the line begins with anything else, it's a line of dialogue.
But maybe you want to add more kinds of "stage directions" later. Audio, effects, and so on. And sooner or later you run out of symbols. So you might want to use just one symbol followed by a word which describes what that symbol means. And if you go so far, then the actual text might not look that special anymore, so it should also be notated by a word. So you might end up with a file format like this:
{ "character": "bob" },
{ "emotion": "happy" },
{ "transition": "fade" },
{ "music": "calm.ogg" },
{ "text": "Pleasure to meet you" }
Wait a minute... that is JSON! A standardized format for hierarchical key/value data. One which Unity can actually parse out of the box. So you don't even need to write your own file parser. You just need to create a plain old C# class with all the fields which you can have in a line of dialogue and then use the method JsonUtility.FromJson
to take a text string and create and instance of that class.