Add JavaScript coverage analysis

What does this MR do?

• configure teaspoon to calculate test coverage of JavaScript files
• publish result as GitLab pages (thanks @ayufan for the idea)

Are there points in the code the reviewer needs to double check?

• the .gitlab-ci.yml

Why was this MR needed?

• test coverage was not calculated for JavaScript files
• JavaScript wants to be under cover

What are the relevant issue numbers?

#19412 (moved)

Screenshots