Support Sticky Scroll (showing markdown headers) in notebooks

[rebornix]

Does this issue occur when all extensions are disabled?: Yes/No

We could explore sticky scroll by showing markdown headers (aligned with TOC and Folding) in notebook editor.

[rebornix]

Here is another scenario that sticky scroll can help with

I was searching for issues created or assigned to me through Find Widget, since the output is large, I can't easily tell what category the output belongs to, whether it's "Open Issues" or "Verifiable Issues". If we have Sticky Scroll headers, it would be much easier to tell.

[Yoyokrazy]

listview top padding support via #178626

[Yoyokrazy]

out of date -- nb sticky scroll is in polish stage. tracking via #188703