Provide option to add created new project folder to multi-root VS Code workspace

[bfitzpat]

Currently when the user activates the "Quarkus: Generate a Maven Project" command, the result is a whole new workbench window with the new project folder as the root. I suspect this is due to the old single folder workspace in VS Code. Now that VS Code has multi-root workspaces (https://code.visualstudio.com/docs/editor/multi-root-workspaces) it would be much less jarring to the user to have the new folder simply added to the workspace rather than opening an entirely separate window. Or at least it would be good to have the option to go either way.