InorderTraversal 
by jiangshan @jiangshanmeta
Implement the type version of binary tree inorder traversal.
For example:
```typescript
const tree1 = {
val: 1,
left: null,
right: {
val: 2,
left: {
val: 3,
left: null,
right: null,
},
right: null,
},
} as const
type A = InorderTraversal // [1, 3, 2]
```
