feat(typescript-types): Add IsEmptyKey<S>

packages/typescript-types/test/is-empty-string.tst.ts

@@ -0,0 +1,8 @@
+import type { IsEmptyString } from '@/is-empty-string.js'
+import { expect } from 'tstyche'
+
+expect<IsEmptyString<''>>().type.toBe<true>()
+expect<IsEmptyString<any>>().type.toBe<true>()
+
+expect<IsEmptyString<'nonempty'>>().type.toBe<false>()
+expect<IsEmptyString<never>>().type.toBe<false>()