* Add ShortenType function

Taken from

  https://github.com/aarzilli/gdlv/blob/master/internal/prettyprint/short.go

with kind permission by @aarzilli.

* Shorten type names in variable values

The variables view in VS Code is a lot easier to read if long type names are
shortened in much the same way as we shorten them for functions in the call
stack view.

We only shorten them in the value strings; the Type field of dap.Variable is
kept as is. Since this only appears in a tooltip, it isn't a problem to have the
full type visible there.