CamelCase 
by Anthony Fu @antfu
`snake_case` 文字列を `camelCase` に変換する `CamelCase ` を実装します。
例えば
```ts
type camelCase1 = CamelCase<'hello_world_with_types'> // expected to be 'helloWorldWithTypes'
type camelCase2 = CamelCase<'HELLO_WORLD_WITH_TYPES'> // expected to be same as previous one
```
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