Add support for extend_file_start and extend_file_end

[hadronized]

In #6837, I added support for things like extend_to_first_nonwhitespace because we had goto_first_nonwhitespace without the extend version. Earlier, I realized we have goto_file_start and goto_file_end but we do not have the extend versions.

Helix has this nice thing of having select mode for users who prefer a mode for selecting, but it also has the possibility to adopt a more traditional Kakoune way of selections by using extend-versions of the regular motions, and I think we should close that gap as much as possible (I personally never use select mode, and have the two wanted actions bound to Gj and Gh in Kakoune).

I’m working on a PR.