subcommand of [string] that returns whether its string argument is a valid Tcl [list]

Accepts both the '''-strict''' and '''-failindex''' ''var'' options, with usual meanings. Note that the '''-strict''' is meaningless as the empty string is always a valid list.

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[AMG]: For Tcl 9, why not remove -strict for [[string is list]]?  It is indeed meaningless.  It's not even good for detecting empty lists, since any string consisting solely of whitespace is an empty list despite not being an empty string.

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