lsp: show function signature, use conform.nvim for formatting code.

config/lsp/conform.nix

@@ -0,0 +1,37 @@
+{ helpers, pkgs, ... }: {
+  plugins.conform-nvim = {
+    enable = true;
+    formatAfterSave = ''
+      {
+        format_after_save = {
+          lsp_format = "fallback",
+        },
+      }
+    '';
+    formattersByFt = {
+      python = [ "black" ];
+      c = [ "clang-format" ];
+      cpp = [ "clang-format" ];
+      cmake = [ "cmake_format" ];
+      nix = [ "nixfmt" ];
+      # Standalone scalafmt is too slow, so we use it as a fallback
+      scala = helpers.mkRaw ''
+        { scalafmt, lsp_format = "prefer" }
+      '';
+      toml = [ "taplo" ];
+      yaml = [ "yq" ];
+      json = [ "jq" ];
+    };
+  };
+
+  extraPackages = with pkgs; [
+    black
+    clang-tools
+    cmake-format
+    jq
+    nixfmt-rfc-style
+    scalafmt
+    taplo
+    yq
+  ];
+}