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 fixes a minor bug where if user hasn't configured their
instaenceUrl (presumably they use the default gitlab.com),
the webview is going to put `null` in the links. This fix
makes sure we use the same logic as the rest of the app.

This rule is critical. We must have a guarantee that when there is
a try/catch around a return statement, we will catch the error.

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.

This is the first step in not swallowing critical errors deep in
the GitService. It allows us to follow the error path for each
public function separately.

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

Also changes button from "No MR" to "Create MR"
See issue #291 for more information.