User can either open the folder or just the file when selecting the problem

[jdneo]

Now if the workspace does not contain the folder that specified in leetcode.workspaceFolder. The extension will ask user to open/add it into the VS Code, which might terminate the execution to generate the problem file -- which hurt the user experience.

We can have two solution for this:

1. Add one more choice such as Just open problem file in editor to only open that file in VS Code.
2. Just open the problem file instead of asking users to open the folder. Then add a command / button that can let users open the folder.