Document and expose the embedAssets export option

[BoosterY]

Context

I noticed that markmap-vscode already supports offline HTML export via the undocumented embedAssets option (added in commit 288d5b1). It works well — setting embedAssets: true in frontmatter or defaultOptions produces a self-contained HTML file with all JS/CSS inlined.

However, this feature is hard to discover:

• It's not listed in the JSON Options documentation
• It's not in the CHANGELOG
• There's no dedicated VS Code setting for it

Proposal

1. Add a VS Code setting like markmap.export.offline (boolean, default false) so users can enable it from the Settings UI without writing JSON in defaultOptions.

2. Document embedAssets in the JSON Options page, or at least mention offline export in the VS Code extension README.

Minor: naming consistency

markmap-cli uses --offline for the same feature. It might be worth aligning the naming — either both use "offline" or both use "embedAssets" — to make it easier for users to discover across tools.

Happy to help

I can open a PR if you think this direction makes sense. The change would be small — mainly adding a configuration entry in package.json and a few lines in exportAsHtml to read it.

[BoosterY]

Adding some context on the use case and how we found this:

Use case: We need to share markmap-generated mind maps as standalone HTML files with others whose environments cannot access external CDNs, so a fully offline HTML is required.

How we found it: We noticed that markmap-cli supports the --offline flag for generating offline HTML, so we wanted to add similar functionality to the VS Code extension. When we started looking into the source code to implement it, we discovered that embedAssets was actually already implemented (commit 288d5b1) — it just wasn't documented or exposed as a VS Code setting.