feat: show changed files for the MR

See merge request gitlab-org/gitlab-vscode-extension!129

The "For current branch" view was completely failing when pipelines
or MRs were disabbled for the current project.

fix: handle disabled pipelines or MRs

See merge request gitlab-org/gitlab-vscode-extension!131

feat(mr review): show changed file diff (API-provided)

See merge request gitlab-org/gitlab-vscode-extension!132

feat: remove experimental features feature flag

See merge request !133

This reverts commit 55cbad7a