Return index of undecodable element in dynamic.list DecodeError path

[johtso]

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https://github.com/gleam-lang/stdlib/blob/d8f046a674197b41e409fec0522a5f6d2d4c7a6d/src/gleam/dynamic.gleam#L320-L329

I think it would be nice to return the index of the offending element instead of just "*"

I'm new to gleam, but I had a little play around trying to implement this and could open a pull request..