Use ?? for Providing Defaults for Results

[smores56]

We want to add a new operator ?? that acts as a Result.withDefault equivalent. Some example usages:

configFile = args.configFile ?? "~/.config/service.json"

dataItems =
    Config.readFile! configFile
   |> Result.try Json.decode
    ?? ["abc", "def", "ghi"]

It should be equal precedence to the |> pipeline operator, such that it can be used after a pipeline. The two above formattings are the only legal formattings.

If a pure handler is used with the new ?? "Result.withDefault" operator, we should raise a warning that the error mapping is unnecessary, since it will always be discarded by ??.