Support F# expression in debugger eval

To support F# expressions in debugger eval (watch/etc) need an F# expression evaluator

A sample evaluator https://github.com/Microsoft/ConcordExtensibilitySamples/wiki/Managed-Expression-Evaluator-Sample

As per info by @gregg-miskelly (debugger team) in https://github.com/OmniSharp/omnisharp-vscode/issues/1257#issuecomment-284833130 i quote directly because good ihmo (is already language agnostic, easier to do than before, debugger team may assist a bit to start)

>For supporting F# expressions - someone would need to develop an F# expression evaluator. There is a sample expression evaluator on https://github.com/Microsoft/ConcordExtensibilitySamples/wiki/Managed-Expression-Evaluator-Sample. It used to be really hard to make a good EE, so the F# team and community never tried to make one (to my knowledge at least). It is certainly not something that someone is going to bang out in a week, but it is way easier than it used to be, so it would be great if someone from the team or community took that on. We are happy to provide some assistance in getting started / when folks get stuck. But it isn't something we have the resources or F# expertise to implement ourselves. Our model is that we provide the language-neutral infrastructure and language teams write the language-specific portions.