[Proposal]: ref readonly method parameters (VS 17.8, .NET 8)

# `ref readonly` method parameters

* [x] Proposed
* [x] Prototype: Done
* [x] Implementation: Done
* [ ] Specification: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-12.0/ref-readonly-parameters.md

> **Note**: this proposal is a follow up to numerous conversations over the years about how `in` arguments are implemented in C#, both here on GitHub and on Discord (with folks like @stephentoub, @jaredpar, @tannergooding, @AaronRobinsonMSFT). To give context and cite the last example, see https://github.com/dotnet/runtime/issues/67445. Opening this so we can properly review it and hopefully start making some progress.

See https://github.com/dotnet/csharplang/blob/main/proposals/csharp-12.0/ref-readonly-parameters.md

## Design Meetings

* https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-04-25.md#ref-readonly-method-parameters
* https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-05-09.md#ref-readonly-parameters
* https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-05-11.md#ref-readonly-method-parameters