Introduce "Files" tab for multi-file snippets
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=231304) </details> <!--IssueSummary end--> As part of the support for multi-file snippets + versioned snippets, we should introduce the relevant tabs as per design: ![Tabs on a snippet](https://gitlab.com/gitlab-org/gitlab/uploads/d1111cf0110f35de0c30b95b30674716/image.png) This issue deals with adding just the "Files" tab in the following form: ![Files tab. The first iteration](https://gitlab.com/gitlab-org/gitlab/uploads/d14d919bb17f3f6a22130cdb77d73dfd/image.png)
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