feat: Support new file contribution - #533

LGTM, and suggest to support full qualified class name in future?

Change the logic to:

1. If current active editor is on the project source path, create a file under the directory of the active editor
2. Otherwise, create an untitled file with a simple new Java class snippet.

Appended a commit. Now if there is no active text editor but having one source path, the source path will be returned as the package path. This is useful in invisible projects.

why Linux-UI failed, is it unstable?

Yes it's unstable

We might need to revisit this feature once we can get the selected file path from the file explorer. See: microsoft/vscode#3553