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

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`.
