Add suggestion diagnostic for enum computed string keys (#42468)

[yeomgahui]

Fixes #42468

This PR adds a suggestion diagnostic when an enum member key uses a computed string like ["Hello"], to encourage clearer and more compatible code.

Although such syntax is allowed in JavaScript, it can reduce readability and lead to subtle bugs. Based on the discussion in #42468, this PR introduces the following suggestion message:

Enum member keys should not use computed strings like ['A']. Use a string literal instead for clarity and compatibility.

This diagnostic is categorized as a Suggestion (code: 18062), so it does not affect existing code and is not a breaking change.

Note: The existing error for truly dynamic expressions (code 1164) remains unchanged.

---

Checklist:

• There is an associated issue: Enum declarations should consistently disallow computed keys #42468
• Code is up-to-date with the main branch
• All tests pass via hereby runtests
• New test case: enumComputedStringSuggestion.ts verifies the suggestion

Feedback welcome 🙌

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