[Proposal]: Partial events and constructors (VS 18.0, .NET 10)

# Partial events and constructors

* Specification: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-14.0/partial-events-and-constructors.md
* Discussion: https://github.com/dotnet/csharplang/discussions/8064

## Summary

Allow the `partial` modifier on events and constructors to separate declaration and implementation parts,
similar to [partial methods][partial-methods-ext] and [partial properties/indexers][partial-props].

```cs
partial class C
{
partial C(int x, string y);
partial event Action MyEvent;
}

partial class C
{
partial C(int x, string y) { }
partial event Action MyEvent
{
add { }
remove { }
}
}
```

[partial-methods-ext]: ./csharp-9.0/extending-partial-methods.md
[partial-props]: ./csharp-13.0/partial-properties.md

## Design meetings

https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-11-02.md#partial-properties
https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-01-22.md#partial-events-and-constructors