fix(scanner): address memory safety and correctness issues in C code

- Add len==0 check in set_contains() to prevent buffer overflow
- Add missing stdlib.h include in scanner.c
- Clear heredoc stack properly in deserialize when length==0
- Ensure NUL termination in delimiter deserialization
- Create alloc.c to define ts_current_* symbols for TREE_SITTER_REUSE_ALLOCATOR

All changes tested with full test suite: 61/61 tests passing.

Addresses PR #1 review comments from CodeRabbit.

src/tree_sitter/alloc.c

@@ -0,0 +1,9 @@
+#include "tree_sitter/alloc.h"
+#include <stdlib.h>
+
+#ifdef TREE_SITTER_REUSE_ALLOCATOR
+void *(*ts_current_malloc)(size_t)        = malloc;
+void *(*ts_current_calloc)(size_t,size_t) = calloc;
+void *(*ts_current_realloc)(void*,size_t) = realloc;
+void  (*ts_current_free)(void*)           = free;
+#endif