feat: Support new file contribution

[jdneo]

1. If there is a active text editor and it's uri belongs to a workspace folder, then generate a new class at the same folder
2. Otherwise, ask user to select target project (if multiple) and package
3. If no project is opened, ask user to create a new project

resolve microsoft/vscode-java-pack#706

[Eskibear]

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

[jdneo]

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.

[jdneo]

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.

[Eskibear]

why Linux-UI failed, is it unstable?

[jdneo]

Yes it's unstable

[jdneo]

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