feat: add 8-bit string test cases across concatenation, contains, count, ends_with, equality, index_of, iterate, iterator_value, join, starts_with, subscript, and to_string

Add comprehensive test coverage for strings containing null bytes (\0) to ensure
all string operations correctly handle 8-bit clean data. Tests verify that
concatenation, substring search, length counting, boundary checks, equality
comparisons, index lookups, iteration, joining, and conversion to string all
treat embedded null bytes as regular characters rather than terminators.

test/string/starts_with.wren

@@ -7,3 +7,8 @@ IO.print("abcd".startsWith("")) // expect: true
 // Non-ASCII.
 IO.print("søméthîng".startsWith("sømé"))  // expect: true
 IO.print("søméthîng".startsWith("some")) // expect: false
+
+// 8-bit clean
+IO.print("a\0b\0c".startsWith("a"))  // expect: true
+IO.print("a\0b\0c".startsWith("a\0"))  // expect: true
+IO.print("a\0b\0c".startsWith("b\0"))  // expect: false