Add zed-jetbrains-air-theme extension

[ArjunKhunti]

This PR adds the zed-jetbrains-air-theme extension to the registry.

It brings a clean, JetBrains-inspired aesthetic to Zed, designed for high readability and a familiar developer experience.

Checks

• The id in extension.toml matches the repository name.
• The version in extension.toml matches the version in the registry manifest.
• The repository includes a valid Open Source License.
• The extension has been tested locally using zed: Install Dev Extension and loads successfully.
• pnpm sort-extensions has been run to ensure alphabetical ordering in extensions.toml and .gitmodules.

[cla-bot]

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[ArjunKhunti]

@cla-bot check

[cla-bot]

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