[Proposal]: extension indexers

# Extension indexers * Specification: * Discussion: https://github.com/dotnet/csharplang/discussions/9855 ## Summary We add the ability to declare extension indexers. ```csharp static class E { extension(...) { int this[int i] { get { ... } set { ... } } } } ``` There are also some other extension improvements we should consider (see [cut list](https://github.com/dotnet/roslyn/issues/76130) from C# 14) ## LDM Meetings * The topic was discussed previously as part of the extension feature in C# 14: https://github.com/dotnet/csharplang/issues/8697