Made the calls to openReplWindow() async

[cfehse]

What has Changed?

• Made the calls to openReplWindow() async to prevent the connection process from being blocked in case the REPL window can not be loaded successfully.

Fixes #399

My Calva PR Checklist

I have:

• Read How to Contribute.
• Made sure I am directing this pull request at the dev branch. (Or have specific reasons to target some other branch.)
• Made sure I am changed the default PR base branch, so that it is not master. (Sorry for the nagging.)
• Tested the VSIX built from the PR (well, if this is a PR that changes the source code.) 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. (For now you'll need to opt in to the CircleCI New Experience UI to see the Artifacts tab, because bug.)
  • Tested the particular change
  • Figured if the change might have some side effects and tested those as well.
  • Smoke tested the extension as such.
• Referenced the issue I am fixing/addressing in a commit message for the pull request.
  • If I am fixing the issue, I have used GitHub's fixes/closes syntax
• Created the issue I am fixing/addressing, if it was not present.
• Updated the [Unreleased] entry in CHANGELOG.md, linking the issue(s) that the PR is addressing.

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