Reset global jumpList on unload

[hrjakobsen]

This PR resets the jumpList that contains document references on unload. This fixes an issue where the extension would attempt to access out-of-range document data when switching between files.

To replicate bug in dev environment:

1. Type (insert mode) module on first line of the Javascript editor
2. Tick "html"
3. Type (normal mode) /module
4. Press n until last match is highlighted
5. Until "html"
6. Press n

[nightwing]

Perhaps we need a way to keep some kind of document id in the jumplist, or keep the jumplist locally, because as far as i can tell with this change destroying an editor by e.g. closing an unfocused tab, will clear the jumplist of the focused tab