Consider using `var` default values as replacement for `CodableDefault`

[kperryua]

Based on forum discussions here, consider using var default values as a replacement for @CodableDefault macros:

struct BlogPost {
    var required: Int
    var items: [String] = []
}

decode(BlogPost.self, from: data) // { "required" : 1, "items": ["hello"] }
// -> BlogPost(required: 1, items: ["hello"])

decode(BlogPost.self, from: data) // { "required" : 1 }
// -> BlogPost(required: 1, items: [])

decode(BlogPost.self, from: data) // { "items": ["hello"] }
// ❌ error, missing field

[kperryua]

The nuances involved here are:

1. The macro will REQUIRE that the code defines an explicit type for the var instead of getting it inferred from the default value.
2. This can only work for vars. lets cannot be modified, even in initializers.
3. This will behave differently than Codable synthesis, which allows default-initialized vars, but doesn't allow those values to be missing from the decoded payload.

[YeoSeongil]

@kperryua
I found the forum discussion really interesting! As an iOS developer, I could relate to a lot of the points raised, particularly around the boilerplate of handling missing JSON fields with optionals.

Issue #2000 seems to require more significant changes including runtime API additions and protocol signature changes, so I'd like to start with #2001 first, as it appears to be achievable with a more targeted change to the macro expansion logic. I'll give it a try!

[kperryua]

Awesome! Thank you so much for the contribution! I'll take a look at your PR.

I think this behavior will evoke some discussion in the eventual review, but I'm happy to have it now in the prototype so we can actually experiment with what it's like to use it in practice.