feat(question): add #59 - Get Optional (#60)

Co-authored-by: Zheeeng <1303154+zheeeng@users.noreply.github.com>

questions/59-hard-get-optional/README.md

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+Implement the advanced util type `GetOptional<T>`, which remains all the optional fields
+
+For example
+
+```ts
+type I = GetOptional<{ foo: number, bar?: string }> // expected to be { bar?: string }
+```

questions/59-hard-get-optional/info.yml

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+difficulty: hard
+title: Get Optional
+tags: 'utils, infer'
+author:
+  github: zheeeng
+  name: Zheeeng
+

questions/59-hard-get-optional/template.ts

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+type GetOptional<T> = any

questions/59-hard-get-optional/test-cases.ts

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+import { Equal, Expect, ExpectFalse, NotEqual } from '@type-challenges/utils'
+
+type cases = [
+    Expect<Equal<GetOptional<{ foo: number, bar?: string }>, { bar?: string }>>,
+    Expect<Equal<GetOptional<{ foo: undefined, bar?: undefined }>, { bar?: undefined }>>,
+]