Support filesFirst in file tree

[chadluo]

Currently when view files in tree mode, it always put folders above files. Would be nice to provide options to sort files above folders, so that you can view files directly under a folder, then files in subfolders. Vscode already supports "explorer.sortOrder": "filesFirst", and this would be similar.

For example in this PR, enable it to show package.json and package.nls.json above api, common and github

[chadluo]

Could be more helpful to support all explorer.sortOrder options