Credit: [@Musisimaru](https://gitlab.com/Musisimaru) (originally
introduced in
[!229](https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/merge_requests/229))

When we dispose commentController, all its therads will get disposed
automatically. We don't ever need to call dispose outside of the
GitLabCommentThread.

Most of the client code was making its own `assert()`, because it
couldn't continue without having an existing repository. By moving the
assert into the `getRepository()` method, we simplify the client code.

VS Code keeps the case. I.e. it  correctly replaces
even getWorkspaceFolders.

The only places where we didn't replace the string were when
we were using the VS Code API (vscode.workspace.workspaceFolders).

We parse the MR version REST API response into hunks. These hunks tell
us what lines have been added in the new file version on the diff.
We then convert these lines in VS Code Ranges and return them as ranges
on which the user might leave a comment.

This is to be reverted in the next MR. This disabling is used instead of
a feature flag. Now the preliminary refactoring can be reviewed
separately.

If the comment changed on the server, but the extension still
shows the old version, we'll prevent the extension from overriding
the server's latest version. We'll warn the user that the
comment has changed since they last saw it.

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.

All logic related to creating vscode.CommentThread and
setting the correct properties has been extracted to one class.

This class will be responsible for resolving threads in the future.

The GitLabComment is going to be responsible for all comment-related
functionality like editing, deleting and reacting.

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.