[FEATURE] - open one main-project with multi sub-projects

[chenjiefly]

@alefragnani Hi, alefragnani! When I forked the vscode-project-manager project and clone it to my local machine, it seems that vscode-project-manager-core is missing on the master branch. Then it's of course failed, when i try to debug the project.

I want to make a contribution about the multi rootPaths, which just means when i open one project named 'project-main', i hope to open multi sub projects(as project-sub1 and project-sub-2) at the same time. the configration may like:

{
"name": "project-main",
"rootPath": [
"$home/develop/project-sub-1",
"$home/develop/project-sub-2"
],
"paths": [],
"tags": [],
"enabled": true
}