Consider adding a "restore" navigation type

[philipwalton]

Currently, for pages that are discarded and then restored, the only way to detect this is via document.wasDiscarded. This is easy enough, but it would be better if these were tracked via a distinct navigationType value on the Metric object.

In Chrome today, these loads receive a navigation type of back_forward, and in Safari they receive a type of navigate, both of which seem not quite applicable, so adding a dedicate type to this library seems like it would be useful to (1) help understand how often this happens, and (2) help determine whether these types of loads have any characteristics that are uncorrelated with other types of navigations.