It's way simpler than you think: When developing and/or building an extension (to try out new features that you've added to the LeoJS codebase itself), you dont build a VSIX, nor go through the packaging process of all the metadata and so on - Here's what to do...
You clone the repo and you open it with vscode as a workspace. (cd into the repo and launch vscode with 'vscode .' the dot means open THIS folder with vscode, not some individual file(s) )
1- run the 'npm install' command in a terminal, (or in vscode's integrated terminal) this will install all the project folder's dependencies.
2- you just press F5 in vscode to compile and run (this will start a new vscode window with the NEW version of the extension running, replacing the existing LeoJS without de-installing it) That new vscode window can be live-debugged, stepped through, and inspected from the original vscode window.
Tada!!
Thanks again for trying out all that stuff ! I know how strange using a new language/framework can feel when coming from an unrelated language&environment paradigm !
Félix