Remove restriction that 'var' variables cannot be const.

[CyrusNajmabadi]

No description provided.

[CyrusNajmabadi]

Proposal is here: https://github.com/CyrusNajmabadi/csharplang/blob/master/proposals/csharp-8.0/const-var.md

[jcouv]

From discussion with Tomas, if we support this for locals, we'll probably want to support this in scripting too (although technically const var x = ...; in script is a field).
But I do not know how to make this work without introducing an unbound recursion (stack overflow). See recent issue: #21223

I'll stop by to share some context.