FlattenDepth 
by jiangshan @jiangshanmeta
Recursively flatten array up to depth times.
For example:
```typescript
type a = FlattenDepth<[1, 2, [3, 4], [[[5]]]], 2> // [1, 2, 3, 4, [5]]. flattern 2 times
type b = FlattenDepth<[1, 2, [3, 4], [[[5]]]]> // [1, 2, 3, 4, [[5]]]. Depth defaults to be 1
```
If the depth is provided, it's guaranteed to be positive integer.
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