List currently installed templates

[jpogran]

The Puppet VS Code extension should be able to list out the currently installed Puppet Content Templates when the user activates the PCT New Content command palate command.

This will display a quickpick where a use can select one template to deploy. A separate ticket can add the input boxes for name and output and run the command.

This ticket will decide whether it makes more sense to execute pct new --list --format json or write typescript code to parse the yaml files.

It should also figure out how to handle --templatepath for a custom path to installed templates. It is possible to not add this at this point while we still figure out how templates are installed.