Add primer primitives extension to recommended VSCode extensions

[siddharthkp]

Marketplace: https://marketplace.visualstudio.com/items?itemName=githubprimer.primer-primitives-autocomplete
Source: https://github.com/primer/primitives-vscode-extension

Rollout strategy

• None; only affects dev environment

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Latest commit: 20e294a

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Pull request overview

This PR introduces a VSCode workspace extensions configuration file to recommend the Primer Primitives autocomplete extension to developers working on this repository. This addition supports the development workflow for contributors working with Primer design tokens and primitives.

Key Changes:

• Creates .vscode/extensions.json with a single recommended extension for Primer primitives autocomplete

[siddharthkp]

Added skip github ci because this is a dev environment only change