Add support for staging changes in the Web IDE

MR: Pending

Description

As a Web IDE user, I want to have a staging area in the source control panel so that I group file changes in different commits.

Acceptance criteria

• Users can stage and unstage changes.

• Staged changes appear in their own resource group.

  

• There are menu actions to stage or unstage changes

  

• Staging operations also appear in the source control panel menu

  

• Users can commit only staged changes

  

• Allow discarding all changes (staged and unstaged).

Implementation plan

• Follow the VSCode's source control provider documentation to learn how to add additional source control features to VSCode: https://code.visualstudio.com/api/extension-guides/scm-provider
• The Web IDE's source control implementation is located in: https://gitlab.com/gitlab-org/gitlab-web-ide/-/tree/main/packages/vscode-extension-web-ide/src/scm?ref_type=heads
• Web IDE virtual file system implementation: https://gitlab.com/gitlab-org/gitlab-web-ide/-/tree/main/packages/web-ide-fs?ref_type=heads
• BrowserFS: https://github.com/jvilk/browserfs