Replace vsce by @vscode/vsce

The `vsce` package has been replaced by `@vscode/vsce`, as announced
in the VSCode 1.74.0 release notes. There should be no functional
changes between 2.7.0 and 2.15.0 for our use case.

See: https://code.visualstudio.com/updates/v1_74#_renaming-of-vsce-to-vscodevsce

Author: koesie10