Allow for uninstalling extensions using the API

[auxves]

There is already a command for installing extensions (workbench.extensions.installExtension), and it would be really nice if there was also a command to uninstall extensions, like workbench.extensions.uninstallExtension. I'm currently working on a PR for code-settings-sync (shanalikhan/code-settings-sync#859), and this is all we need to completely switch to using the API from the CLI.

[sandy081]

A command workbench.extensions.uninstallExtension to uninstall an extension

Usage:

vscode.commands.executeCommand('workbench.extensions.uninstallExtension', 'extensionId');