fix: allow empty ranges at the end of a sequence for lists and strings

Previously, empty ranges were only permitted at index zero on an empty sequence.
This change extends the `calculateRange` function to allow empty ranges at the
end of a non-empty sequence, enabling `list[3...3]` and `list[3..-1]` to return
an empty slice. Updated the range validation logic to check `range->from == *length`
instead of requiring a zero-length sequence, and added test cases for both lists
and strings to verify the new behavior.

test/core/list/subscript_range.wren

@@ -33,3 +33,7 @@ System.print(list[3...-6]) // expect: [d, c, b, a]
 // An empty range at zero is allowed on an empty list.
 System.print([][0...0]) // expect: []
 System.print([][0..-1]) // expect: []
+
+// An empty range at the end is allowed on a list.
+System.print([1, 2, 3][3...3]) // expect: []
+System.print([1, 2, 3][3..-1]) // expect: []