Render Timeline Headers in Table of Contents

[ChrisRisner]

Is your feature request related to a problem? Please describe.
Not necessarily a problem.

Describe the solution you'd like
When I use a Timeline shortcode in an article, I'd love to be able to specify if timeline headers should show up (ideally as subitems) in the Table of Contents.

Describe alternatives you've considered
I could not use the timeline feature but definitely like how it displays things.

Additional context
From what I could tell in briefly looking, header elements in the timeline are rendered the same as what the TOC renderer is looking for (i.e. as H2s) but I believe I'd need them to render as H3s (to be subitems) and be rendered before the TOC is done in order for it to work. I haven't had time to examine if delayed loading or another method of building the TOC would make more sense.