New Lint: empty_enum_variants_with_brackets

[ARandomDev99]

This PR:

• adds a new early pass lint that checks for enum variants with no fields that were defined using brackets. Category: Restriction
• adds relevant UI tests for the new lint.

Closes #12007

changelog: New lint: [`empty_enum_variants_with_brackets`]

[rustbot]

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[Jarcho]

Thank you. @bors r+

[bors]

📌 Commit 5f17188 has been approved by Jarcho

It is now in the queue for this repository.

[bors]

⌛ Testing commit 5f17188 with merge 7f185bd...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Jarcho
Pushing 7f185bd to master...