Rule Request: Prevent moot unwrapped variable in guard/if-let

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

I countered some cases in my codebase where I unwrap a variable in a guard/if-let, but then don't use it directly.

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.

It prevents moot code.

2. Provide several examples of what would and wouldn't trigger violations.

This:

guard
	let asset = asset, // asset is not used
	let date = asset.creationDate
else {
	return
}

search(withQuery: date)

Could simply be:

guard
	let date = asset?.creationDate
else {
	return
}

search(withQuery: date)

4. Should the rule be configurable, if so what parameters should be configurable?

No

5. Should the rule be opt-in or enabled by default? Why?
   See README.md for guidelines on when to mark a rule as opt-in.

Having moot code is not harmful, so I guess it could be opt-in.

Start with the proposed guard and if-let examples in this issue, then review README.md's opt-in rules guidance. The work is complete when SwiftLint can identify the described moot unwrapped variables and the rule's opt-in status is defined.