Support checkboxes in headings

Use case or problem

When building nested lists with checkboxes, sometimes you want to have higher level categories that are easy to spot: this is the goal of headings.
If checkboxes in heading would work and render as they do not in headings, it would be useful here.

Proposed solution

The following syntax should create a checkbox, and when the checkbox is checked, should strike through the line:

# - [ ] This is a heading with an unchecked checkbox
#### - [X] This is a heading with a checked checkbox

It should work with any level of heading. The whole heading should get strikethrough when the checkbox is checked, like it is done currently in non-heading cases.

Current workaround (optional)

I use the syntax above, but the checkbox is not displayed, neither is the strikethrough.

Related feature requests (optional)

This is briefly mentionned too in this thread: Support checkboxes in tables - #40 by ohreallyhmmm

I think the proposed format is unlikely to happen — I don’t know of any Markdown app that supports anything like that.

But headings in list items are already supported (tho maybe not fully — they might not appear in the outline, etc).

- ## Heading in a list item

So headings in task list items would be consistent with that.

- [ ] ## Heading in a task list item

Currently this doesn’t work in Live Preview and looks janky in Reading View.