Rename `GetValueForOption` and `GetValueForArgument` to `GetValue` and add these methods to `InvocationContext`

[jonsequitur]

In order to make ParseResult.GetValueForArgument and ParseResult.GetValueForOption easier to find and use, the suggestion was made in API review to add them to InvocationContext.

We should also simplify the naming to GetValue. This should be done on ParseResult as well as on SymbolResult.