Add a topic for Squirrel!

[TuberAsk]

Please confirm this pull request meets the following requirements:

• [x] I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• [x] I am not the sole author or employee of a company who created either the topic I am modifying/adding or the collection entry I am modifying/adding.

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Which change are you proposing?

• [ ] Suggesting edits to an existing topic or collection
• [x] Curating a new topic or collection
• [ ] Something that does not neatly fit into the binary options above

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Curating a new topic or collection

• [x] I've formatted my changes as a new folder directory, named for the topic or collection as it appears in the URL on GitHub (e.g. https://github.com/topics/[NAME] or https://github.com/collections/[NAME])
• [] My folder contains a *.png image (if applicable) and index.md
• [] All required fields in my index.md conform to the Style Guide and API docs: https://github.com/github/explore/tree/main/docs

Squirrel is really underated, I think it's a great language and if we can make a topic for it, that would have been great. I think it would be cool if more users would notice Squirrel. Squirrel does not use tons of RAM, it is embeddable and lightweight.