fix: replace strncpy with memcpy and increase error message buffer size

Replace strncpy with memcpy in readBuiltInModule to avoid potential string termination issues
when copying module source code, and increase ERROR_MESSAGE_SIZE from 60 to 80 to provide
more room for compiler error messages with long variable names.

src/vm/wren_compiler.c

@@ -48,7 +48,7 @@
 // is kind of hairy, but fortunately we can control what the longest possible
 // message is and handle that. Ideally, we'd use `snprintf()`, but that's not
 // available in standard C++98.
-#define ERROR_MESSAGE_SIZE (60 + MAX_VARIABLE_NAME + 15)
+#define ERROR_MESSAGE_SIZE (80 + MAX_VARIABLE_NAME + 15)
 
 typedef enum
 {