Inline clientside callbacks in Dash for R #135

As noted in plotly/dash#956 and plotly/dash#967, it would be very convenient for app developers who wish to leverage JavaScript callbacks to compose these directly within the callback method. One approach might be to pass the JS directly as a string:

app$callback(
  output('out-component', 'value'),
  params=list(input('in-component1', 'value'), input('in-component2', 'value')),
  clientsideFunction(
    "
    function(largeValue1, largeValue2) {
        return someTransform(largeValue1, largeValue2);
    }
    "
    )
)

The namespace / function name can be determined in a fashion similar as what's done in DashPy (plotly/dash#967), or not, with the _dashprivate_ prefix.

This feature should be reasonably straightforward to implement in Dash for R, and it would help improve developer ergonomics if it were available.