Introduce a bespoke `global-server.nix` module. This is less DRY, but
much simpler.
The `lsp.servers."*"` options are different enough from the other
`lsp.servers.<name>` options that it is simpler to just declare them
separately.
Now that we have a dedicated `global-server.nix` module, we no longer
need to split the normal server module into `server.nix`+`server-base.nix`
This partially reverts f2e96b67a3
