Add control to open file shown in "replace preview" editor when performing project-wide find/replace

While performing a project-wide find/replace, opening an occurrence from the search sidebar opens a "replace preview" diff editor. This view is not editable, so if the user would like to edit the displayed file, the only current way to do so is manually either from the explorer sidebar or the quick-open palette.

Per @roblourens' suggestion in #35323, "There should be a working Open File button that always appears, git repo or not, in the replace diff editor."