[Proposal]: Final initializers

# Final initializers

* Specification: [proposals/final-initializers.md](https://github.com/dotnet/csharplang/blob/main/proposals/final-initializers.md)
* Discussion: https://github.com/dotnet/csharplang/discussions/6591

## Summary

Final initializers are a proposed new kind of member declaration that runs at the end of an object's initialization—after constructors, object and collection initializers, and `with` expressions.

Early notes refer to these as "validators."

```cs
public class Range
{
public int Min { get; init; }
public int Max { get; init; }

init
{
if (Max < Min)
{
throw new InvalidOperationException("Max must not be less than min.");
}
}
}
```

## Design meetings

[2020-04-27: Initial decision on `init {`...`}` syntax](https://github.com/dotnet/csharplang/blob/main/meetings/2020/LDM-2020-04-27.md#primary-constructor-bodies-and-validators)