Add browser context test of mangrove.js to CI

[espendk]

Is your feature request related to a problem? Please describe. In relation to #220 we encountered an issue where mangrove.js didn't work in the browser (due to a Node specific dependency), but we only realized this after publishing mangrove.js; When we tried to upgrade the front-end to that version, it failed at load time due to an exception from mangrove.js.

Describe the solution you'd like We should have a test in the CI flow that checks whether mangrove.js can run in the browser. We might want to run all mangrove.js test suites there, but at a minimum we should ensure that it can be loaded without error.

cypress.io might be of use here (Johan's suggestion), but I haven't researched other options yet.

Describe alternatives you've considered None yet - this needs to be thought through.

Additional context The package we had problems with was node-config.