Check style on all workspace source files

[anissen]

This issue is simply an issue to bring attention to the following tasks/issues already mentioned in the README:

Limitations: it doesn't see violations in your project files until you open them
TODO: Check on all the workspace

One approach to handle this would be to add a command palette operation to vscode-checkstyle. This operation would do a one-time check of all source files in the workspace (similarly to running haxelib run checkstyle -s src).