Dynamically import translations as json files and fixing tests

[r59q]

This is the PR associated with the proposal #398

We suggest moving to svelte-i18n for translations (MIT licence). This will enable:

• A move to a JSON file for each language along with a standard approach to supporting parameters in strings that >platforms like Crowdin understand (ICU message syntax)
• Bundle splitting so each additional language doesn’t bloat the app size
• Low impact on the existing code - only initialization code and one translation usage needs to change

This is still a draft, a change to the test suite is needed before merging

Update

It is however ready to be merged, but to the top-level await some of the tests have to be temporarily skipped in the suite, unfortunately. See #407 for more info. practically speaking, the test coverage of these tests have not been changed for a while (microbit-interfacing). So hopefully the down-time of these tests won't result in any issues before we can fix them.