Restricted Linter options type

[JoshuaKGoldberg]

Fixes #1167

Wasn't sure where unit tests for this would go, if you do want to test constructor logic.

[jkillian]

👍 We can probably get away without a test here for now, it would break some of the full CLI tests if it didn't accept valid input. Do you think we should also add a check for == null and throw an error then?

[JoshuaKGoldberg]

Yeah, that sounds like a good thing to add.

Do you consider any option fields mandatory? If so, instead of checking for typeof it might make sense to check for their existence.

[jkillian]

Strangely enough, it looks like ILinterOptions.formatter must be specified, while everything else can be left undefined. This seems more like a bug to me than anything, so let's not worry about it - I think ILinterOptions.formatter should be made optional in the long run.

On the other hand, if everything is optional, passing null as the current options isn't unreasonable. Maybe we should just modify computeFullOptions to handle the case where the whole options object is null and to provided a default value for formatter if not provided?

[JoshuaKGoldberg]

Thoughts on this draft? Is there some source of truth for what formattershould default to?

[jkillian]

let's not add the || "" here, it's fine if this stays null. (An empty string would get treated as the current directory which could lead to unexpected behavior)

[jkillian]

Left my thoughts above @JoshuaKGoldberg, feel free to disagree on any of them!

[jkillian]

Beautiful! This turn out very nice