Support formatting & organize import for multiple files

[CsCherrYY]

Currently we can only do formatting or organize import for the current file, is it possible to do these actions for multiple files? e.g., when the user selects a folder in the file explorer, we can offer an option like "format files" in the menu.

Eclipse supports such feature:

[jdneo]

Related: https://github.com/redhat-developer/vscode-java/issues/1107