Introduce the index selector (non-negative)

[V0ldek]

Tracking issue for the list Index Selector [n], where n is a positive constant. Support for negative (selecting from array end) is tracked at TODO.

• Parser Parse the index selector (non-negative) #60
• Compiler Compile the index selector (non-negative) #61
• Engine (recursive) Introduce the index selector (non-negative) into the recursive engine #62
• Engine (main) Introduce the index selector (non-negative) into the main engine #63

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