Rule request: [discarded_async_let_error] async let discards thrown errors

New Issue Checklist

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

New rule request

When using async let on a throwing function, with or without try, or, inside or outside of a do/catch, the thrown error is discarded and this might be unexpected.
Wrapping in a do { } catch { } adds a false sense of effectiveness. Maybe there should be a unneeded_do_catch rule, too.
Either the do/catch should be removed or the async let _ = try ... should be replaced with try await ... while avoiding try?.

try? converts a thrown error to an optional and still discards the error.
try! avoids the described problem with other risks.

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.
   This helps simplify and correct code because either the do/catch was written and is not needed or the programmer has made a logical mistake by discarding the error by forgetting to try await the result. Or, the programmer should use try await instead of async let. I'm not sure what the community thinks.

2. Examples:

do {
   async let resultOne = MyThrowingFunction() // Trigger
   async let resultTwo = try MyThrowingFunction() // Trigger
   // async let resultThree = try? MyThrowingFunction() // Potential trigger
} catch {
   print("This never gets printed for any of the above")
}

Task {
   async let resultOne = MyThrowingFunction() // Trigger
   async let resultTwo = try MyThrowingFunction() // Trigger
   // async let resultThree = try? MyThrowingFunction() // Potential trigger
}

do {
   async let resultOne = MyThrowingFunction() // Won't trigger because do/catch + await
   try await resultOne
} catch {
   print("This gets printed")
}

do {
   async let resultTwo = try MyThrowingFunction() // Won't trigger because do/catch + await
   try await resultTwo
} catch {
   print("This gets printed")
}

do {
   async let resultTwo = try MyThrowingFunction() // Probably should trigger since try? await still discards the error
   try? await resultTwo
} catch {
   print("This gets printed")
}

Task {
   async let resultOne = MyThrowingFunction() // Trigger
   async let resultTwo = try MyThrowingFunction() // Trigger because no do/catch even though it is awaited.
   async let resultThree = try? MyThrowingFunction() // Trigger

   try await resultTwo  
}

4. Should the rule be configurable, if so what parameters should be configurable?
   Configurations could include checking try? and/or try!.

5. Should the rule be opt-in or enabled by default? Why?
   Enabled by default because it catches potential logical errors by the programmer and should not have many occurrences.

The issue names no implementation files, entry points, or tests. Start by reviewing the async let examples and deciding the intended handling of discarded errors, try?, try!, and do/catch cases. Done means the rule's scope and default or configuration behavior are agreed and the listed trigger and non-trigger cases are covered.