chore: add cobertura test coverage

Description

This MR copies the setup from Language Server to surface coverage reports in GitLab MR review page

• https://gitlab.com/gitlab-org/editor-extensions/gitlab-lsp/blob/d2ab450a2faca5ce3453a923c46507a6da66fcf0/jest.config.ts#L4-6
• https://gitlab.com/gitlab-org/editor-extensions/gitlab-lsp/blob/d2ab450a2faca5ce3453a923c46507a6da66fcf0/.gitlab-ci.yml#L73-77

How has this been tested

I added a simple change and then reverted it (see the commit history):