Add Vitest snippets extension

[jwerre]

This pull request adds a new snippet-only extension for Vitest, the Vite-native test framework.

Given Vitest's popularity in the JavaScript and TypeScript communities, these snippets are intended to help developers write tests more quickly and efficiently.

This PR includes:

The extension repository (https://github.com/jwerre/vitest-snippets) added as a submodule under extensions/.

A new entry for [vitest-snippets] in the root extensions.toml file, pointing to the new submodule.

The extension provides snippets for common Vitest globals and functions, such as describe, it, test, expect, vi.fn, vi.mock, and more.

Please let me know if any changes are required for this to be merged.

[cla-bot[bot]]

We require contributors to sign our Contributor License Agreement, and we don't have @jwerre on file. You can sign our CLA at https://zed.dev/cla. Once you've signed, post a comment here that says '@cla-bot check'.

[jwerre]

@cla-bot check

[cla-bot[bot]]

The cla-bot has been summoned, and re-checked this pull request!

[jwerre]

Please fix the CI errors, you can follow https://zed.dev/docs/extensions/developing-extensions for this. Also, please make sure that the extension works locally. Thanks!

@MrSubidubi This should be ready to go now