WIP: Terminal Jack-in

[PEZ]

What has Changed?

Currently Calva abuses the Task system for controlling the Jack-in process. But tasks are really something that the users should be able to specify. I fear that at some point jack-in will stop working if do not do something about this.

In this PR I instead use a custom/fake terminal, through the new ExtensionTerminal API together with a ”regular” child_process. It makes much more sense to do it this way, IMO.

The cons I think are:

• Might be opening up the can of worms with command line quoting again. This needs to be tested on some different flavours of especially Windows setups.
• Someone might be using an old VS Code that doesn't support this API. (I'm ready to disregard this completely, but. it is there.)

Fixes #635

My Calva PR Checklist

• Read How to Contribute.
• Directed this pull request at the dev branch. (Or have specific reasons to target some other branch.)
• Made sure I have changed the default PR base branch, so that it is not master. (Sorry for the nagging.)
• Updated the [Unreleased] entry in CHANGELOG.md, linking the issue(s) that the PR is addressing.
• Tested the VSIX built from the PR (so, after you've submitted the PR). You'll find the artifacts by clicking Show all checks in the CI section of the PR page, and then Details on the ci/circleci: build test. NB: There is a CircleCI bug that makes the Artifacts hard to find. Please see this issue for workarounds.
  • Tested the particular change
  • Figured if the change might have some side effects and tested those as well.
  • Smoke tested the extension as such.

The Calva Team PR Checklist:

Before merging we (at least one of us) have:

• Made sure the PR is directed at the dev branch (unless reasons).
• Read the source changes.
• Given feedback and guidance on source changes, if needed. (Please consider noting extra nice stuff as well.)
• Tested the VSIX built from the PR (well, if this is a PR that changes the source code.)
  • Tested the particular change
  • Figured if the change might have some side effects and tested those as well.
  • Smoke tested the extension as such.
• If need be, had a chat within the team about particular changes.

Ping @PEZ, @kstehn, @cfehse, @bpringe