Add support for "firstInSection" and "lastInSection" when adding menu items

When `position` is `"firstInSection"` or `"lastInSection"`, `relativeId` must be a command in the section where you want to add the item.

This follows the same rules as the HTML menus:

* The section must be in the specified menu. `ERR_NOT_FOUND` is returned if the section is not in the specified menu.
* A "section" is delimited by the top/bottom of the menus or a separator.

There will be a pending brackets pull request to enable this functionality from within Brackets.