feat: Add plaintext format detection

[Yahiewi]

This PR adds format detection to the library which can be useful to automatically convert a plaintext file to a notebook without having to select what the format is each time.
This is especially useful for example in the Specta standalone app, which automatically converts files without prompting the user to choose the format. With these changes, Specta and other extensions can correctly convert plaintext files without format input from the user.

The main code changes are:

• src/detect.ts (new): The main important function here is detectFormat(text, ext).
• src/index.ts: exports new functions parseFormat and serializeFormat.
• test/detect.test.ts (new): Tests different cases using the newly created functions.

[github-actions]

🎉 This PR is included in version 1.8.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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