markup/goldmark: Support extras extension

[jmooring]

Enables inclusion of these HTML elements in Markdown:

• Inserted Text (++inserted++)
• Mark Text (==marked==)
• Subscript (H~2~O)
• Superscript (1^st^)

Default configuration:

markup:
  goldmark:
    extensions:
      extras:
        insert:
          enable: false
        mark:
          enable: false
        subscript:
          enable: false
        superscript:
          enable: false

[jmooring]

@bowman2001 Building from this PR should just work. Note the configuration structure in the comment above; one extension with four options.

See documentation preview:
https://deploy-preview-2555--gohugoio.netlify.app/getting-started/configuration-markup/#extras-extension

Also see description in the Hugo Goldmark Extensions README:
https://github.com/gohugoio/hugo-goldmark-extensions?tab=readme-ov-file#extras-extension

And a one page test site with some simple tests:

git clone --single-branch -b hugo-github-issue-12459 https://github.com/jmooring/hugo-testing hugo-github-issue-12459
cd hugo-github-issue-12459
hugo server

Looks good to me. Next steps:

• Review of extension code (bep)
• Update extension test matrix (hugo-goldmark-extensions#19) (bep)
• Tag an extension release (v0.1.0) (jmooring)
• Update this PR to reflect the extension version (jmooring)

---

Portability with other apps/systems:

	Inserted text	Mark	Subscript	Superscript
GitHub	no	no	no	no
GitLab	no	no	no	no
Obsidian	no	yes	no	no
Typora	no	yes	yes	yes
VS Code	no	requires extension	no	requires extension

[bowman2001]

Thanks a lot! Your integration test is way more concise than mine was.

[bep]

This looks good now, I will merge this early next week and get it into the same release as the "pages from data" feature. I'm just holding it just in case I need to do a patch release for some other reason.