Multi-root Workspaces

[dsanders11]

Current form of the extension is built on the assumption that there's a single workspace folder, and that it's the Electron checkout (either src or src/electron). Things "Mostly Work" if you add a second workspace folder after, as long as the first workspace folder has the Electron checkout.

Provide better support for finding an Electron checkout that's not the first workspace folder. To avoid unnecessary complexity, add a constraint that there can only be one Electron checkout workspace folder. Perhaps allow a special case where you have both src and src/electron as separate workspace folders.