Migrate from jsdom to vitest browser mode

[43081j]

Nobody should really be using jsdom today, since headless browsers are fast to launch and fast to run tests.

vitest has very good support for browser tests for a while now, too.

Reasons to migrate:

• jsdom doesn't represent any users (in that no user uses jsdom)
• it is far behind browsers in terms of standards, and always will be
• vitest browser mode is often as fast as using jsdom

[ghostdevv]

If you're doing it with Svelte then @spences10 made a good website with information on migrating: https://sveltest.dev/