Add VS Package with commands

* Use a standard toolbar for diff commands
* Update next/previous functionality to show the popup in addition to moving the caret

~~**IMPORTANT:** This change is *mostly* complete, but the implementation of `EditorDiffMarginControl.HandleCustomPopupPlacement` needs to be updated to account for the position of the mouse when the user clicks on a diff. The behavior I envision is:~~

* ~~If the vertical position of the mouse is located within the vertical span of the diff whose popup is being shown, offset the vertical position of the popup to match the position of the mouse during the custom popup placement.~~
* ~~Otherwise, use the behavior as currently implemented.~~

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This change adds most of the required functionality to implement #8. While Visual Studio will allow users to bind keyboard shortcuts to these actions, execution of the actions from any location other than the toolbar buttons for the diff popup will result in calls to the `GitDiffMarginPackage` methods, which are not yet implemented.