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

Some features do not work after GitLab version 13.5.
This update alerts the user that they are using an outdated
version.

Multiple paces will need to search through all the MR changes and find
the correct diff. This commit extracts a method to do that.

We crated a fragment that contains all note information. We then use
this fragment for fetching discussions and for returning note from the
createNote mutation. We also had to fix the webview integration test
where the fake API response must now contain the note.

The real reason for this simplification is summed up in the following
line:

```
export type GqlNote = GqlTextDiffNote | GqlImageNote | GqlOverviewNote;
```

We can now refer to notes by GqlNote instead of having to specify
generic arguments. That makes the code easier to read and it will allow
us to return GqlNote from the createNote mutation.

Excluding the position from overview notes was a small optimization.
Removing the condition simplifies the code at the cost of small
overhead (currently roughly 20 requests per hour).
After this change we'll always fetch the diff position.

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.

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

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

Extracting configuration related logic came up in reviews multiple times.
Also, the VS Code logic returns missing properties as null.
This behaviour is not ideal because null function arguments overrdie the
default function arguments.

Related discussions:
- https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/merge_requests/115#note_432956679

this limits the blast radius of existing GitLab issue
https://gitlab.com/gitlab-org/gitlab/-/issues/298827

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