ensure appropriate version of dotnet-interactive is present

[brettfo]

extension.ts will now ensure that an appropriate version of dotnet is installed, or else it will acquire one for the user. If one couldn't be acquired, it is a fatal error and nothing can be done.

A similar check is also done for dotnet-interactive, but it currently only continues if a valid version is present and errors otherwise. An upcoming PR will be to acquire a local tool version of it if necessary. If the appropriate version is not present, an error is shown in the bottom corner or the window:

The minimum good version of dotnet-interactive is the one with the recent hover text updates from a few days ago. That version has not yet been published to NuGet.org, so when we do publish again, it should be at least that version. A workaround is that all locally-built arcade versions of the tool are allowed (*-dev).

Also included are some other minor improvements like not returning from execute() until either CommandHandled or CommandFailed has returned. This makes the notebook UI in VS Code much nicer.

N.b., the changes to interactiveClient.ts are mostly whitespace, so disabling that in the GitHub view is recommended for that file.

[brettfo]

@colombod I've added a dispose() method to KernelTransport so that the process can be killed when VS Code disposes the notebook.