Feat: implement keyof about Enum

[sunrabbit123]

Description:

// @strictNullChecks: true
// @declaration: true

enum E { A, B, C };

type K15 = keyof E;  // "toString" | "toFixed" | "toExponential" | ...

BREAKING CHANGE:

Related issue (if exists):

[github-actions]

Thank you for the PR!
Commit: 31ce0f5

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[kdy1]

Thank you!