feat: add range subscripting support for strings with CalculatedRange helper

Extract range validation logic from list_subscript into a reusable calculateRange function that handles inclusive/exclusive bounds, negative indices, and direction stepping. Add CalculatedRange struct to wren_core.h and implement string subscripting via the new helper, including comprehensive test coverage for forward, backward, negative, and half-negative ranges with proper error messages for out-of-bounds and non-integer values.

test/string/subscript_range.wren

@@ -0,0 +1,34 @@
+var string = "abcde"
+IO.print(string[0..0]) // expect: a
+IO.print(string[1...1] == "") // expect: true
+IO.print(string[1..2]) // expect: bc
+IO.print(string[1...2]) // expect: b
+IO.print(string[2..4]) // expect: cde
+IO.print(string[2...5]) // expect: cde
+
+// A backwards range reverses.
+IO.print(string[3..1]) // expect: dcb
+IO.print(string[3...1]) // expect: dc
+IO.print(string[3...3] == "") // expect: true
+
+// Negative ranges index from the end.
+IO.print(string[-5..-2]) // expect: abcd
+IO.print(string[-5...-2]) // expect: abc
+IO.print(string[-3..-5]) // expect: cba
+IO.print(string[-3...-6]) // expect: cba
+
+// Half-negative ranges are treated like the negative value is fixed before
+// walking the range.
+IO.print(string[-5..3]) // expect: abcd
+IO.print(string[-3...5]) // expect: cde
+IO.print(string[-2..1]) // expect: dcb
+IO.print(string[-2...0]) // expect: dcb
+
+IO.print(string[1..-2]) // expect: bcd
+IO.print(string[2...-1]) // expect: cd
+IO.print(string[4..-5]) // expect: edcba
+IO.print(string[3...-6]) // expect: dcba
+
+// An empty range at zero is allowed on an empty string.
+IO.print(""[0...0] == "") // expect: true
+IO.print(""[0..-1] == "") // expect: true