Extend Text_Sub_Range with From_End_While

[wdanilo]

This task is automatically imported from the old Task Issue Board and it was originally created by Radosław Waśko.
Original issue is here.

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While most constructors of Text_Sub_Range have pairs that work both from the beginning and end of the string, the While constructor does not have such a variant.

However it may be useful to take or drop some characters form the end of the string based on a predicate. Thus we should add a dual of While, called From_End_While which selects a range of characters satisfying the given predicate, but looking from the end of the string.

    ## Select characters from the end of the string while the predicate returns
       `True`.
    type From_End_While (predicate : (Text -> Boolean))