[Proposal]: `left join` and `right join` query expression clauses

# `left join` and `right join` query expression clauses

* Specification: https://github.com/dotnet/csharplang/blob/main/proposals/left-right-join-in-query-expressions.md
* Discuss this proposal here: https://github.com/dotnet/csharplang/discussions/8892

## Summary
[summary]: #summary

Introduce `left join` and `right join` clauses into LINQ query expression syntax, e.g.:

```c#
from student in students
left join department in departments on student.DepartmentID equals department.ID
select new { student.Name, department?.Name }
```

## Design meetings

* https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-02-12.md#left-right-join-in-query-expressions