New `optional_map` rule

New Issue Checklist

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

New Rule Request

Please add an optional_map rule.

It would change:

• if let nonOptional = optional {a} else {b} to optional.map {a} ?? b
• if let nonOptional = optional {a} to optional.map {a}.
• if let optional {a} else {b} to optional.map {a} ?? b
• if let optional {a} to optional.map {a}.

(same for all variants of if var …)

If a can be nil, then flatMap (instead of map) would be used.

It could also apply to switches that are the equivalent of if-else, but I assume those are rare.

Maybe a rule already exists to convert from such a switch to an equivalent if-else

Maybe this rule could be limited to assignments / returns / etc., or have parameters to limit its scope.

I imagine that it should be opt-in, because maybe not everyone will agree with my preference for the concision of optional.map, but if there was a consensus that optional.map should be used (at least in certain circumstances), then it could be a default rule.

No files, tests, or entry points are named. Start by locating SwiftLint's existing rule implementations and tests, then resolve the scope and opt-in behavior for the listed if-let and if-var forms, including when to use map versus flatMap; done means the agreed forms are detected and covered by tests.