Add allowedTags to HTMLContentTransform request

[compulim]

Changelog Entry

Added

• Added HTML content transformer middleware, in PR #5338 and #5347, by @compulim
  • Configure HTML sanitizer via request.allowedTags

Description

Added request.allowedTags to enable custom elements story through HTML content transform middleware.

Design

Background: the HTML sanitizer is the very last function in the HTML content transform middleware.

One story of HTML content transform middleware is to convert, say, <a> into <my-link>.

This PR enables the middleware to allowlist additional tag names for the HTML sanitizer. It can also enable denylist and observe the allowed tag names.

Specific Changes

• Updated HTMLContentTransformMiddleware to include new allowedTags
• Updated the "code block copy button" middleware to use this feature
• Moved default sanitizer options to the caller of the middleware, i.e. useTransformHTMLContent() hook

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)