sections: [
        "one"
        "two"
        "three"
    ]

and

    sections: [
        "one"
        ["two" "three"]
    ]

exists already and is named rebol (and their derivatives)

In your first example you’re just using newlines as delimiters, the second is allowing either newline or comma.

Or taking a Lisp perspective, atoms separated by white space.

That's how I do it in Civet (the CoffeeScript of TypeScript). It's a very small amount of complexity to add to the parser for a ton of convenience when using the language. https://github.com/DanielXMoore/Civet

Ruby has an interesting syntax for defining arrays of literal values.

  irb(main):001:0> sections = %w(one two three)
    => ["one", "two", "three"]

I know and it works on multiple lines too. However I never use it. Requirements change, refactoring happens, one of those literals becomes an interpolated string and all of a sudden I have to rewrite all the array. Furthermore %w() is just not nice to look at IMHO.