feat: replace public byteAt with private byteAt_ and add O(1) bytes.count

Remove the public `byteAt(_)` method from String and its ByteSequence wrapper, renaming it to the private `byteAt_(_)` to eliminate redundancy with `.bytes[_]`. Add a native `byteCount_` primitive on String and expose it as `count` on StringByteSequence, enabling O(1) byte length queries. Update all test files to use the new `.bytes` subscript API and add a dedicated `count.wren` test for the new functionality.

test/core/string_byte_sequence/count.wren

@@ -0,0 +1,15 @@
+// Simple.
+IO.print("".bytes.count) // expect: 0
+IO.print("123".bytes.count) // expect: 3
+
+// UTF-8.
+// Bytes:
+//        123456789
+// Chars: sø mé ஃ
+IO.print("søméஃ".bytes.count) // expect: 9
+
+// Null bytes.
+IO.print("\0\0\0".bytes.count) // expect: 3
+
+// Invalid UTF-8.
+IO.print("\xef\x00".bytes.count) // expect: 2