fix(onlineManager): always initialize with `online: true` (#5714)

* fix(onlineManager): always initialize with `online: true`

instead of relying on navigator.onLine, because that indicator is broken AF in chrome

https://bugs.chromium.org/p/chromium/issues/list?q=navigator.online

* docs: document subscribe methods

* docs

* test: fix types in tests

setting to undefined is now illegal, so we have to reset to `true`, which is the default now

* fix(tests): switch from mocking navigator.onLine to mocking onlineManager.isOnline

* fix: offline toggle in devtools

it should now be enough to set online to true / false, without firing the event, because we always set & respect that value. we don't override the event handler, so real online / offline events might interfere here

* chore: fix tests

with the implementation of onlineManager, where we default to `true`, we need an explicit `'offline'` event to get it to false; otherwise, switching back to true doesn't change the state, so subscribers are not informed

* chore: prettier write

* chore: fix eslint

* chore: delete an old, flaky test that doesn't test much