实现`StartsWith`,接收两个string类型参数,然后判断`T`是否以`U`开头,根据结果返回`true`或`false`
例如:
```typescript
type a = StartsWith<'abc', 'ac'> // expected to be false
type b = StartsWith<'abc', 'ab'> // expected to be true
type c = StartsWith<'abc', 'abcd'> // expected to be false
```
