Use newer version of JSDOM

[koesie10]

To prepare for upgrading to @vscode-elements/elements, we need a newer version of JSDOM that better supports web components. This upgrades to a newer version of JSDOM by using a custom Jest environment that is available in Jest 30.

[Copilot]

Pull Request Overview

This PR updates the JSDOM version to support web components by switching to a custom Jest environment.

• Updates jest.config.ts to use the custom environment
• Introduces a new jest-environment-jsdom.ts file
• Updates the dead code detection script to ignore the new environment file

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

File	Description
extensions/ql-vscode/src/view/jest.config.ts	Updates testEnvironment to point to the custom Jest environment file
extensions/ql-vscode/src/view/jest-environment-jsdom.ts	Adds a custom Jest environment implementation using a newer JSDOM
extensions/ql-vscode/scripts/find-deadcode.ts	Excludes the newly added Jest environment file from dead code detection

Files not reviewed (1)

• extensions/ql-vscode/package.json: Language not supported

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[robertbrignull]

LGTM if this helps us move forwards with the vscode/elements migration. Maybe it's not ideal being on an alpha version but given we only use it for testing it can hopefully only cause stability issues for us and not for users 🤷🏼

[robertbrignull]

Looking at jest-environment-jsdom it seems we're going jsdom version 20 to 26.

Looking in the changelog I can't immediately see any references to the fields that weren't defined and were causing errors, but it could be I missed it or it's from another updated dependency 🤷🏼

[koesie10]

I believe what we're looking for is in version 23.1.0: "Added an initial implementation of ElementInternals, including the shadowRoot getter and the string-valued ARIA properties." This is what was giving errors when using some @vscode-elements/elements components (after fixing a missing CSSProperties.replaceSync definition).