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 is an optimisation where we check the local git repository
and only when the file in given version doesn't exist there
we'll call the GitLab API to retrieve the file content.

Side panel logic is now enclosed with try/catch and the new error
reporting is used to capture and log the error.

Signed-off-by: NMassimeddu Cireddu <massimeddu@gmail.com>

Closes #22

Closes #20

Closes #25