[Proposal]: Target-typed inference for constructor calls

# Target-typed inference for constructor calls

* Specification: https://github.com/dotnet/csharplang/blob/main/proposals/inference-for-constructor-calls.md
* Discussion: https://github.com/dotnet/csharplang/discussions/9629

## Summary
[summary]: #summary

Generic type inference is extended to 'new' expressions, which may infer type arguments for the newly created class or struct, including [from a target type](target-typed-generic-type-inference) if present. For instance, given:

```csharp
public class MyCollection : IEnumerable
{
public MyCollection() { ... }
...
}
```

We would allow the constructor to be called without a type argument when it can be inferred from arguments or (in this case) a target type:

```csharp
IEnumerable c = new MyCollection(); // 'T' = 'string' inferred from target type
```

## Design meetings

* https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-08-18.md#type-inference
* https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-08-20.md#target-type-inference-improvements
* https://github.com/dotnet/csharplang/blob/main/meetings/2026/LDM-2026-08-19.md#type-parameter-inference-for-constructor-calls