feat: implement environment table for vm

include/vm/fzl_vm.h

@@ -5,6 +5,7 @@
 #include <compiler/utils/fzl_const_pool.h> // NOTE: move outside compiler scope to common
 #include <stddef.h>
 #include <stdint.h>
+#include <vm/fzl_vm_env.h>
 
 // VM descriptor
 typedef struct
@@ -26,7 +27,8 @@ typedef struct
   size_t ctop;
 
   // Environment stack
-  // TODO: implement `estack` and `etop`
+  fzl_vm_env_t estack[256];
+  size_t etop;
 } fzl_vm_t;
 
 // Allocate FZL VM