There is no such built-in mechanism. Past experience is the people doing this would either use an outline-specific app, e.g. OmniOutliner, or add the formatting on export. More recently Tinderbox preview mechanism means this long-form mark-up can be viewed within the app.
SoMe question arise (by way of unpacking adjustments, not critique!):
How many different formats are proposed and based on what standard.
Is the level mark-up tied to current position (i.e. changes if the base container is moved), or does it inhere from a designated root container?
How is this handled in export? As literal characters or in a form that might assist other outliners to ingest it?
I’ve done things like this in the past, but they’re a bit of a kludge and normally end up not being suitable because one or more of the above the points was added as a requirement after the work was done.
Another ui/human factors issue occurred to me and that is are we doing this simply because it gives us the comfort of looking like the UI of some other tool. For instance, if I write such a list in Word, I need to see the styling as it’s a WYSIWYG tool - you are (in default view) looking at the ‘printed’ form of the work. But what if, as becoming more common, I write in a styling agnostic fashion such as mark-up.
On reflection, I think the latter helps makes sense of the questions above. I note that having admittedly fallen down just such “wouldn’t it be cool if” paths myself, in the past, and finding that after finishing some byzantine code that it wasn’t clear what/which problem I’d solved.
That said, I’ll address some potential moving parts of a solution in a separate reply.