modules/lsp: declare a stripped down servers."*" option

`"*"` is effectively a meta server, where shared config/defaults can be
set.

It shouldn't have options like `activate` or `package` which relate to
"real" servers. Therefore, we'll use `server-base.nix` directly, instead
of the full `server.nix` module.

modules/lsp/server-base.nix

@@ -0,0 +1,55 @@
+# Usage: lib.importApply ./server-base.nix { /*args*/ }
+{
+  displayName ? "the language server",
+  settings ? null,
+}:
+{ lib, ... }:
+let
+  inherit (lib) types;
+in
+{
+  options = {
+    enable = lib.mkEnableOption displayName;
+
+    settings = lib.mkOption {
+      type = with types; attrsOf anything;
+      description = ''
+        Configurations for ${displayName}. ${settings.extraDescription or ""}
+      '';
+      default = { };
+      example =
+        settings.example or {
+          cmd = [
+            "clangd"
+            "--background-index"
+          ];
+          root_markers = [
+            "compile_commands.json"
+            "compile_flags.txt"
+          ];
+          filetypes = [
+            "c"
+            "cpp"
+          ];
+        };
+    };
+
+    # NOTE: we need a warnings option for `mkRenamedOptionModule` to warn about unexpected definitions
+    # This can be removed when all rename aliases are gone
+    warnings = lib.mkOption {
+      type = with types; listOf str;
+      description = "Warnings to propagate to nixvim's `warnings` option.";
+      default = [ ];
+      internal = true;
+      visible = false;
+    };
+  };
+
+  imports = [
+    # TODO: rename added 2025-04-30 (during the 25.05 cycle)
+    # The previous name `config` was introduced 2025-04-28 (during the 25.05 cycle)
+    # Because the previous name `config` never made it into a stable release,
+    # we could consider dropping this alias sooner than normal.
+    (lib.mkRenamedOptionModule [ "config" ] [ "settings" ])
+  ];
+}