refactor: extract shared clipboard module from webidl-clipboard

[marcoscaceres]

Summary

• Extracts the copy button SVG and creation logic from webidl-clipboard.js into a shared clipboard.js module
• webidl-clipboard.js now delegates to createCopyButton(".idlHeader")
• Prepares for CDDL (feat(core/cddl): add CDDL support #5201) and other code block types to reuse the same copy functionality

Split out from #5201 per Sid's request.

Test plan

• Modified Web platform tests — no observable behavior change (pure refactor)

[Copilot]

Pull request overview

This PR refactors the existing WebIDL copy-to-clipboard button implementation by extracting the shared copy button creation/copy logic into a reusable core/clipboard.js module, enabling reuse by other code block processors (e.g., upcoming CDDL support).

Changes:

• Introduces src/core/clipboard.js with a shared createCopyButton(headerSelector) helper.
• Updates src/core/webidl-clipboard.js to delegate to createCopyButton(".idlHeader").

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/core/webidl-clipboard.js	Replaces inline copy-button creation/copy logic with a call to the shared helper.
src/core/clipboard.js	Adds a shared copy-button factory and click handler that copies <pre> contents excluding a header element.

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[sidvishnoi]

Lgtm. Let's use copilot suggestion to add aria-label though. Rest is good.

[Copilot]

Lgtm. Let's use copilot suggestion to add aria-label though. Rest is good.

Addressed in 2005e2a.

The shared copy button now sets aria-label from the configurable title value, and WebIDL uses "Copy IDL to clipboard" so tooltip and accessible name stay specific.

Screenshot: