add vim mode via query param

[Rich-Harris]

Adding ?vim=true will enable Vim mode in the playground and tutorial. Ultimately it might be nice to have some UI around this, but I'm not sure what that looks like given it needs to work for both places.

In an ideal world we wouldn't need to mess around with compartments, we would just add the vim plugin to default_extensions directly. Unfortunately await import(...) at the top level doesn't work in Safari, because Safari is a bad piece of software, so we have to do this dance instead.

closes #570