Support non-Latin heading slugs in non-Markdown markup

Background

MarkupHeadingAnchorFilter, introduced in !238950 (merged) and !241282 (merged), generates heading IDs and anchor links for non-Markdown markup (Org-mode, RST, MediaWiki, etc.). It uses ActiveSupport::Inflector.parameterize to derive slugs from heading text.

Problem

Since parameterize strips all non-Latin characters, headings written in non-Latin scripts fall back to the tag name (e.g., h1, h2). The resulting anchors give no indication of which heading they point to.

This is inconsistent with Markdown rendering, where heading slug generation is Unicode-aware.

Solution

This MR replaces parameterize with Unicode-aware slug generation, modelled after the Markdown parser Comrak's anchorize().

References

• Support table of contents button for org-mode f... (#600361 - closed)

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