VS FSI should provide syntax highlighting

(As requested as a result of #2156 being merged)

Related to #2161 - not sure whether they should be merged into a larger 'Recreate VS FSI as a Roslyn interactive window' so we can take advantage of the features also used by Python Tools.

The FSI tool window in VS should provide syntax highlighting as the CLI now does. It would require either transmitting the layout tags from FSI to Visual Studio, or have the tool window parse and colourise the text in the tool window itself (the latter is probably easier).