Support stable anchor links in bespoke template

[yhatt]

bespoke template has become to support links for the stable anchor that has assigned to the element in the slide content. (e.g. Slugified Markdown headings, supported by Marp Core v3.6.0)

By this PR, users can jump to the slide page based on contents even if not used a link to the page number such as [](#3).

Slugs

# Title

[Jump to Hello](#hello)

---

## Contents

- ...
- ...

---

## Hello

Example

In general Markdown document, a link of "Jump to Hello" can actually jump to "Hello" heading. By the change in this PR, the HTML with bespoke template also can jump to the slide page 3.

Named anchor (with --html option)

[Jump to closing slide](#closing-slide)

---

...

---

<a name="closing-slide"></a>

Thanks!

bespoke template also supports the anchor defined through <a name="..."></a>. It's useful when there is not any Markdown headings in the slide.

Stable anchor in fragmented lists

[Show up to "Item 2"](#item-2)

* ### Item 1
* ### Item 2
* ### Item 3

It may be a rare case, but bespoke template can handle the stable anchor within the fragmented list too.

Related

• Naming slides (stable anchor links) marp#161