Determine if the given letter is an alphabet.
import type { Equal, Expect } from '@type-challenges/utils'
type cases = [
Expect<Equal<IsAlphabet<'A'>, true>>,
Expect<Equal<IsAlphabet<'z'>, true>>,
Expect<Equal<IsAlphabet<'9'>, false>>,
Expect<Equal<IsAlphabet<'!'>, false>>,
Expect<Equal<IsAlphabet<'😂'>, false>>,
Expect<Equal<IsAlphabet<''>, false>>,
]