`InteractiveDocument.GetInputFields` should detect the variable name instead of the prompt text when used with `#!set`

[jonsequitur]

This issue was originally opened here: jonsequitur/dotnet-repl#90.

The underlying cause is that #!set --name should override the usual way in which input token field names are assigned. Microsoft.DotNet.Interactive.Documents does not take this special treatment into account.