addon_folders.ts: assign scheme to URIs created for mockups

[kiraacorsac]

Correctly constructs URI objects that are passed to functions related to searching addon files.

Additionally, vscode dependency is updated to a non-depricated version, compatible with the same engine version, and typescript is updated to a version that is compatible with that version.

Small changes in addon_folder.ts are to make the new typescript version happy.

Closes #146