Add ability to specify toolchain

[Emik03]

Feature Description

Common use of REPLs is to experiment, so naturally it would be a nice addition to be able to choose a locally installed toolchain with a CLI argument "--toolchain", primarily for testing out .NET 8 and a new feature, such as primary constructors (which as of the time of writing is in Preview and experimental).

I'm not sure if the C# eval package even supports the ability to do so without recompiling it altogether. If it is supported though, I'd love to see this be implemented. Thanks!

[waf]

We don't have a specific, versioned system for this, but we do generally try to roll forward to whatever the latest .NET version is, and enable preview features.

Primary constructors do work for me despite CSharpRepl targeting .net7.0 -- are they working for you in the current version of CSharpRepl?

[Emik03]

Huh, I recall distinctly that primary constructors didn't work for me, but upon testing it again they now do. I suspect it's either from updating csharprepl or because I updated the net7.0 toolchain just now. I incorrectly assumed net8.0 was where all of the experimental features were.