Add onboarding tasks for the TW fullstack engineer role

Adds more engineering-related tasks to the fullstack engineer onboarding issue.

The additional tasks are mostly copied from a few other onboarding issue templates listed below, but should all be relevant for the TW role.

• https://gitlab.com/gitlab-com/people-group/people-operations/employment-templates/-/blob/main/.gitlab/issue_templates/onboarding_tasks/department_core_development/role_backend_engineer.md
• https://gitlab.com/gitlab-com/people-group/people-operations/employment-templates/-/blob/main/.gitlab/issue_templates/onboarding_tasks/department_core_development/role_frontend_engineer.md
• https://gitlab.com/gitlab-org/frontend/onboarding/-/blob/master/.gitlab/issue_templates/frontend_onboarding.md?ref_type=heads