useless-fallthrough: do not report fallthrough before default

[anupriyakkumari]

Description

This PR improves the useless-fallthrough rule to allow an empty fallthrough before a default case in switch statements, especially for enum-like constants.

Background:

• The exhaustive linter requires all enum-like constants to be covered in switch cases.
• revive currently requires a default case in switches.
• Go doesn't support exhaustive enums natively, so a default: case is often necessary even when all constants are handled.
• Combining a default with a case that only contains fallthrough is invalid.
• This change prevents the rule from flagging such valid fallthroughs used to account for exhaustive switches.

To Reproduce

type SomeType int

const (
    One SomeType = iota
    Two
    Three
    Unknown
)

func CheckParsedVal(i SomeType) int {
    switch i {
    case One:
        return 1
    case Two:
        return 2
    case Three:
        return 3
    case Unknown:
        fallthrough
    default:
        return -1
    }
}

Expected behavior

• The fallthrough before the default case should not be reported as useless.
• This allows exhaustive switches to use a default case gracefully without requiring comments or lowering confidence scores.

Checklist

• Added tests to cover the new behavior
• Code follows the coding style of the repository
• GitHub Action build passes

Footer

Closes #1538