Introduce startEdittingComment and cancelEdittingComment
commands that will open and close the VS Code comment
text area. The cancel command also resets the comment body
to the value retrieved from the API.

This is marked as refactor, because we don't want this commit to have
a line in the changelog. It's an uncomplete feature.

A simple implementation of the git.clone command for the configured
instances.
It also includes a credential provider that can clone the repository
without user input using the access token

See merge request gitlab-org/gitlab-vscode-extension!172

Relates to #222

This change starts fetching position for each discussion and then
it displays the discussions on diffs using the native VS Code API