Allow public fields to be set by OptionParser

[Insomniak47]

Currently because public fields are member access but aren't properties when they are turned into PropertySelectors they will throw an exception even though it's a valid thing to want to do. We should:

• Create a FieldAccess type that handles access to fields
• Add code to determine the nature of the expression type and construct the correct one
• Plumb them into the different places that need it

alternatively, the property expression type could handle both. a C# field isn't a C# property, but an object property is logically an object field. I have no strong preference, but since it's already exposing Func/Action and those are going to be needed to read/write the field it seems like it would make abstract layer above cleaner if we just pretended it's the same thing

[Insomniak47]

Yeah possibly, I don't think it ends up as a ton of work either way so I'll probably just poke at it over time. This was more for tracking