Work around an exception when testing for shortcuts in `keydown` event handlers,
if the handler is called with an argument that is not a `KeyboardEvent`, and is
missing the expected `key` property. The workaround has been added to
`matchShortcut` because it is easier to do there than remember for each
`keydown` event handler, although there is a risk that other code in these
handlers could get caught out by the same issue.

I was able to reproduce this problem locally in Chrome 118 when using autofill
to insert text into the search input field (see comments), though there might be
other causes as well.

This should fix https://hypothesis.sentry.io/issues/3987386810/?project=69811