[css-nav-1] Improve the name of window.navigate

Migrated from https://github.com/WICG/spatial-navigation/issues/120
Originally created by @frivoal on Wed, 17 Oct 2018 05:21:10 GMT

Previously in #70, we discussed whether window.navigate() was a great name for this API or not. It seems like it could be confused with navigating to a new URL, so some better name would be good.

However, no suggestion so far seems to work:

• focusNavigate() or other names invoking focus are wrong, because it can also scroll things instead of focusing them
• directionalNavigate() or spatialNavigate() or similar are also problematic, because this method should be possible to extend to sequential navigation by passing it a "prev" or "next" argument, and that does not work with such names.

So, leaving this issue open to see if we can do better.