DX-1128: localise syntax-highlighter util + registry

[jamiehenson]

Part of DX-1128. Localises the syntax-highlighter utility + language registry.

What

src/utilities/syntax-highlighter.js + syntax-highlighter-registry.js, vendored from @ably/ui. highlight.js (+ highlightjs-curl) promoted to direct deps (were transitive via @ably/ui). CodeBlock rewired to the local util. No icons involved.

Behaviour

No visual change intended.

Stack (DX-1128)

	PR	Role
	#3409	Design foundation — vendor @ably/ui tokens + reset/core CSS (merged)
	#3410	Local cn + heights utils
	#3411	Docs-native Icon component (Ably glyphs only; Heroicons imported directly)
	#3412	Presentational components (Badge, Button, LinkButton, FeaturedLink, Status)
	#3413	Interactive components (SegmentedControl, TabMenu)
	#3414	ProductTile + product data (leaf)
▶	#3415	syntax-highlighter util + registry
	#3416	Code + CodeSnippet — completes the lift; prunes dead CSS + drops @ably/ui Tailwind scan

#3410–#3416 are a linear stack, each building on the previous; #3409 has merged to main. ▶ = this PR.

Testing

• Load the review app.
• Check a sufficient sample of pages load with no visual regressions.

🤖 Generated with Claude Code

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch dx-1128/docs-syntax-highlighter

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[jamiehenson]

Where this is used (review aid):

• syntax-highlighter util + registry → src/components/Markdown/CodeBlock.tsx (rewired to the local util here), which renders every fenced code block in MDX via src/components/Markdown/MarkdownProvider.tsx.
• From DX-1128: localise Code + CodeSnippet (completes the visual lift) #3416 on, Code + CodeSnippet consume it too.

Note: the util + registry are now .ts (typed), not the vendored .js — highlightjs-curl (untyped) gets an ambient decl in src/globals.d.ts.

~ 𝒞𝓁𝒶𝓊𝒹𝑒