[Proposal]: Always available extension methods

# Always available extension methods

* Specification: https://github.com/dotnet/csharplang/discussions/8623
* Discussion: https://github.com/dotnet/csharplang/discussions/8623

## Summary
[summary]: #summary

Currently it is impossible to make sure that an extension method is always in scope, except by putting it in the global namespace. We add a step to lookup where we look for extension methods in the same namespace as the receiver type.

## Design meetings

* https://github.com/dotnet/csharplang/blob/master/meetings/2020/LDM-2020-11-11.md#always-available-extension-methods