Prevent JavaScript from navigating the page within Via

When a user clicks on a link in Via it opens the link target in a new tab that is not proxied by Via: #9

But when the page is navigated by JavaScript this still navigates the original tab and does not leave Via. We need to intercept these programmatic navigations and make them open in a new, non-proxied tab like normal link navigations do.

There are multiple ways that JavaScript can navigate the page. This is the master issue for all of them:

- [ ] Programmatically clicking a link: https://github.com/hypothesis/viahtml/issues/181
- [ ] Programmatically submitting a form
- [ ] Single-page navigations using `window.history` (These don't actually load a new page, they just update the location bar.)
- [ ] Assigning a new value to `window.location` or `window.location.href`

Slack threads:

* https://hypothes-is.slack.com/archives/C1M8NH76X/p1621418996025700