[6.0] Fix regression where external links are rendering as plaintext

[mportiz08]

• Explanation: Fixes regression where external links render as plaintext
• Scope: Impacts pages with links to external sources
• Issue: rdar://133444687
• Risk: Low, focused change to make JS logic more strict when stripping link data
• Testing: Updated unit tests, manually tested pages with internal and external link content, verified that known external links are still purposefully rendered as plaintext in the scenario from #878
• Reviewer: @hqhhuang
• Original PR: #886

[mportiz08]

@swift-ci test