modules/lsp/servers: move to dedicated file/dir

Move the code related to the `lsp.servers` option into a dedicated module,
cleaning up `modules/lsp/default.nix`.

modules/lsp/servers/server.nix

@@ -0,0 +1,84 @@
+# Usage: lib.importApply ./server.nix { /*args*/ }
+{
+  name ? "the language server",
+  package ? null,
+  settings ? null,
+  pkgs ? { },
+}@args:
+{
+  lib,
+  name,
+  config,
+  ...
+}:
+let
+  inherit (lib) types;
+  displayName = args.name or "the language server";
+  packageName = package.name or (lib.strings.removePrefix "the " displayName);
+in
+{
+  options = {
+    enable = lib.mkEnableOption displayName;
+
+    name = lib.mkOption {
+      type = types.maybeRaw types.str;
+      description = ''
+        The name to use for ${displayName}.
+        Supplied to functions like `vim.lsp.enable()`.
+      '';
+      # Use the supplied attr name, or fallback to the name module-arg
+      default = args.name or name;
+      defaultText = args.name or (lib.literalMD "the attribute name");
+    };
+
+    activate = lib.mkOption {
+      type = types.bool;
+      description = ''
+        Whether to call `vim.lsp.enable()` for ${displayName}.
+      '';
+      default = config.name != "*";
+      defaultText = lib.literalMD ''
+        `true`, unless the server's `name` is `*`
+      '';
+      example = false;
+    };
+
+    package = lib.mkPackageOption pkgs packageName {
+      nullable = true;
+      default = package.default or package;
+      example = package.example or null;
+      extraDescription = ''
+        ${package.extraDescription or ""}
+
+        Alternatively, ${displayName} should be installed on your `$PATH`.
+      '';
+    };
+
+    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"
+          ];
+        };
+    };
+  };
+
+  imports = [
+    ./server-renames.nix
+  ];
+}