panelset: Fully support nested panelsets

[gadenbuie]

Fixes #86

Also use panelset ID instead of generated ID, if available, and improve the panelset ID generation.

Nested panelsets are now supported in general with hand-rolled markup. In R Markdown or Quarto documents, nested panelsets are possible using the fenced div syntax:

# Nested fenced divs

::: {.panelset #outer}

## One

Pariatur dolor est pariatur excepteur ad consectetur anim veniam sit ad laboris ad commodo Lorem.

::: {.panelset #inner}
### One A

Et duis nulla sunt eu elit est incididunt qui. Occaecat ullamco ad ut ipsum quis sunt dolor aute amet eiusmod. Ad mollit voluptate consequat exercitation culpa dolore culpa adipisicing.

### One B

Officia ea eu ad sunt ullamco consectetur nisi cillum culpa consequat sit esse ad. Reprehenderit incididunt eiusmod est sint amet ad pariatur reprehenderit in officia. Ipsum exercitation ad quis non ex mollit dolore.
:::

## Two

Exercitation proident consequat velit officia irure incididunt veniam.
:::

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6d6f779) 52.51% compared to head (e80a01e) 52.51%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #194 +/- ## ======================================= Coverage 52.51% 52.51% ======================================= Files 20 20 Lines 794 794 ======================================= Hits 417 417 Misses 377 377 ```

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