General negative indexes for an array

[ABridoux]

Describe your idea briefly.
Allow to target the last nth element of an array, rather than only the last one.

Describe the solution you'd like
Something like scout read array[-2] would allow to read the element at index 3 if the array count is 5.

Remarks

• The negative index can no longer be used to set a value to a last index in an array. As this feature is already offered with the add command, it will be removed.
• To append a value rather than insert it at the last valid index, the add command should use the count | [#] element to specify that the value should be added and not inserted.