add nix ftplugin

after/ftplugin/nix.lua

@@ -0,0 +1,2 @@
+vim.opt_local.tabstop = 2
+vim.opt_local.shiftwidth = 2

after/lsp/rust_analyzer.lua

@@ -0,0 +1,19 @@
+return {
+    settings = {
+        ["rust-analyzer"] = {
+            files = {
+                -- .direnv dirs symlink into full nixpkgs source trees in the nix
+                -- store; without excluding them rust-analyzer follows the symlinks,
+                -- tries to watch/index ~100k dirs, hits the inotify watch limit and
+                -- never finishes loading.
+                excludeDirs = {
+                    ".direnv",
+                    "libkiwoom/.direnv",
+                    "crates/libkiwoom-sys/.direnv",
+                    "target",
+                    "node_modules",
+                },
+            },
+        },
+    },
+}