Also renamed BinarySource to ArtifactSource in anticipation of
nightlies installation that requires downloading
not a binary itself but .vsix package, thus generalized
to `artifact` term

The new name seems much simpler and it doesn't limit
this config value only to downloading the server binary.
Thus we wouldn't need to create another config
properties to handle other downloads whatsoever.
Anyway, I believe (heuristically) that most of the users
would want to set "askBeforeDownload": false once
and never bother clicking on the notification again
(because otherwise there is no big point in installing rust-analyzer if it cannot install the server)

…matklad