[Proposal]: Target-typed inference for type patterns

# Target-typed inference for type patterns

* Specification: https://github.com/dotnet/csharplang/blob/main/proposals/inference-for-type-patterns.md
* Discussion: https://github.com/dotnet/csharplang/discussions/9631

## Summary
[summary]: #summary

Generic type inference is extended to type patterns, which may omit a type argument list when it can be inferred from the pattern input value. For instance, given a declaration `Option intOption`, instead of:

```csharp
if (intOption is Some some) ...
```

You can simply write:

```csharp
if (intOption is Some some) ... // 'Some' inferred from the type of 'intOption'
```

## Design meetings

* https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-08-27.md#type-inference-in-patterns