Add stepped slices for arrays and strings

Normalized profile

What this task asks for

Add stepped slice parsing, slicing, and assignment for arrays and strings with rune-correct indexing. New stepped range behavior (`value[start:end:step]`) must work in both directions. A step of `0` must raise an error that starts with: `slice step cannot be 0`. A non-numeric `start` in array/string slices must keep the existing index-operator error format. Non-numeric `end` or `step` values in ranges must keep the existing numeric-range error format. If a string range selects zero indexes, any non-empty replacement string must raise a size-mismatch error. Assignment error strings must match exactly.