feat: add alejandra formatter for nix (#817)

* feat: add `alejandra` formatter for `nix`

* add `--quiet` argument to `alejandra` formatter

Co-authored-by: Jose Alvarez <j.alvarez11@icloud.com>

lua/null-ls/builtins/formatting/alejandra.lua

@@ -0,0 +1,20 @@
+local h = require("null-ls.helpers")
+local methods = require("null-ls.methods")
+
+local FORMATTING = methods.internal.FORMATTING
+
+return h.make_builtin({
+    name = "alejandra",
+    meta = {
+        url = "https://github.com/kamadorueda/alejandra",
+        description = "The Uncompromising Nix Code Formatter",
+    },
+    method = FORMATTING,
+    filetypes = { "nix" },
+    generator_opts = {
+        command = "alejandra",
+        args = { "--quiet" },
+        to_stdin = true,
+    },
+    factory = h.formatter_factory,
+})