feat(mr review): display whether discussion is resolved or not

See merge request gitlab-org/gitlab-vscode-extension!193

refactor: introduce separate classes for threads and comments

See merge request gitlab-org/gitlab-vscode-extension!190

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.

The testing responsibility is now strictly divided. The tree_view test
is responsible for validating querying lists of issues/MRs and
showing these lists as items in the tree_view.

The new mr_review test is responsible for all interactions with the MR
item, including showing changed files when clicked, fetching
discussions and any future review functionality.

This commit splits the "shows project merge requests assigned to me with
changed files" test in two and all other tests just moved around
unchanged.

chore(ci variables): update the ci_variables.json

See merge request gitlab-org/gitlab-vscode-extension!191

refactor: use union types to better represent discussions

See merge request gitlab-org/gitlab-vscode-extension!189

Developer documentation review and update.

See merge request gitlab-org/gitlab-vscode-extension!188

The testing strategy has been written before we started
writing tests. Now after months of writign tests, lot of the
processes changed.

We migrated to TS and introduced new concepts since the last update.

This is a snippet from a book Effective TypeScript:

WARNING
The examples in this item prefix type names with I or T solely to
indicate how they were defined. You should not do this in your code!
Prefixing interface types with I is common in C#, and this convention
made some inroads in the early days of TypeScript. But it is considered
bad style today because it’s unnecessary, adds little value, and is not
consistently followed in the standard libraries.

Implement git clone command

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

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

docs: add architecture.md

See merge request gitlab-org/gitlab-vscode-extension!187

This document explains how the current code is structured and it also
hints at what changes are we planning to do to the extension structure
in the near future.

feat: suport detached pipelines

See merge request gitlab-org/gitlab-vscode-extension!186

After implementing detached pipelines, this method is only used within
the gitlab_service module.

We had to change the command for openning pipeline in the browser
to support detached pipelines.

All the data we need are in the API response that lists pipelines.

Prefer detached pipelines and only report normal branch pipeline when
there isn't any detached pipeline (MR pipeline).

We also change the logic to fetch MR and pipeline at the same time
because we need to know MR anyway to be able to get detached
pipeline.

Refactoring status_bar to use both pipeline and MR from the response
is too difficult to squeeze in this change. We'll have to do it later.

This reduces the need for readers to keep a variable
in their head to read 50 LOC.

By relying on an exception, we are able to remove several checks
for failed project fetching. This also saves one extra call to
the /project endpoint. Additionally, this commit replaces a long
if statement with an early return.

The `pipelineGitRemoteName` setting is used to say:
"Use this remote for getting pipelines"
This purpose of this configuration is to mark a fork. Then
you can read issues and MRs from the main project and
pipelines from your fork.

The service can be made simpler by moving the pipeline specific remote
out of the service and exposing a more general method.

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.

test: increase integration test timeout

See merge request gitlab-org/gitlab-vscode-extension!184