Create a `SnakeCase` generic that turns a string formatted in **camelCase** into a string formatted in **snake_case**.
A few examples:
```ts
type res1 = SnakeCase<"hello">; // => "hello"
type res2 = SnakeCase<"userName">; // => "user_name"
type res3 = SnakeCase<"getElementById">; // => "get_element_by_id"
```
