Ship default keybindings for 'Move file up' and 'Move file down' in the project explorer

[baronfel]

Users expect Alt+Up/Down to move files in a project based on experiences in other editors. We should ship this keybinding when in the context of a given F# project, like other extensions do:

The when should be something like: fsharp.project.any && solutionExplorerHasFocus && solutionExplorerHasFileInProjectSelected, logically speaking.