Implement ref switch expressions

[Rekkonnect]

This draft PR is approved by @CyrusNajmabadi in offline conversations. Feature and PR review is not expected to happen before the release of C# 12.

The feature's spec is dotnet/csharplang#7352

Currently handled

Features

• Syntax parsing
• Variable assignment
• Direct return in method body
• Direct assignment to the ref
• Direct compound assignment to the ref
• Ref and value escaping based on scope
• Readonly ref kind

Handy features

• IDE suggestions for ref within arms
  • Always suggest ref, even when the switch expression does not have a ref on the left of it
  • This already works, this PR does not need to address this

Errors

• Not using ref returned from the expression
• Not all arms have ref or throw
• C# preview required
• ref on switch expression not returning ref (better hint about the feature)