Add macro for VecMapper

[janniksam]

I use it in quite a few places:

[gfusee]

I haven't decided yet the future of mapper macros (excepted @Storage and @mapping which I will keep)

I personally prefer the Swift DSL syntax because it allows to intuitively create mappers with multiple keys:

VecMapper(baseKey: "myMapper") {
    firstDynamicKey
    secondDynamicKey
    etc....
}

The DSL way is fully typed, doesn't rely on any context and thus doesn't constraint the dev, you can declare it even outside a struct. Any error in writing a mapper this way will result in a clear error message, while this is not the case with a macro (both in Swift and Rust). Moreover, there is no magic code using a DSL, you create the function and you know what it does: instantiating a mapper, this is important for the dev experience