[d3m1d0v]

Add HeadingAnchors to @gravity-ui/markdown-viewer/extensions to copy links to headings with existing unique IDs.

• Show a # button on hover and keyboard focus, with copy feedback. Hide built-in Diplodoc heading controls while mounted.
• Let applications build URLs with getHref({id, element}) and handle navigation with onClick(event, {id, element}).
• Provide --g-md-heading-anchor-background-color for colored containers. Add usage examples to the viewer demo, editor preview, and README.

Summary by Sourcery

Add configurable heading anchor buttons for copying and navigating links to document headings.

New Features:

• Add the HeadingAnchors extension for copying links to headings with unique IDs and configurable URL and navigation callbacks.

Enhancements:

• Show heading anchor controls on hover and keyboard focus with localized copy feedback, while preserving heading content and supporting folding controls.
• Support custom heading selection and colored-container backgrounds through --g-md-heading-anchor-background-color.

Documentation:

• Document the heading anchor extension, callbacks, customization options, and cleanup behavior.
• Add heading-anchor examples to the viewer demo and editor preview.

Tests:

• Add unit, type, package-import, and visual coverage for copying links, callbacks, localization, focus behavior, folding headings, and colored containers.

[gravity-ui]

[gravity-ui]