Add alejandra for nix (#4435)

- Fixes https://github.com/dense-analysis/ale/issues/4434

doc/ale-nix.txt

@@ -2,6 +2,24 @@
 ALE Nix Integration                                           *ale-nix-options*
 
 
+===============================================================================
+alejandra                                                   *ale-nix-alejandra*
+
+g:ale_nix_alejandra_executable                 *g:ale_nix_alejandra_executable*
+                                               *b:ale_nix_alejandra_executable*
+  Type: |String|
+  Default: `'alejandra'`
+
+  This variable sets the executable used for alejandra.
+
+g:ale_nix_alejandra_options                       *g:ale_nix_alejandra_options*
+                                                  *b:ale_nix_alejandra_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be set to pass additional options to the alejandra fixer.
+
+
 ===============================================================================
 nixfmt                                                         *ale-nix-nixfmt*
 

doc/ale-supported-languages-and-tools.txt

@@ -378,6 +378,7 @@ Notes:
   * `nimlsp`
   * `nimpretty`
 * nix
+  * `alejandra`
   * `nix-instantiate`
   * `nixfmt`
   * `nixpkgs-fmt`

doc/ale.txt

@@ -3094,6 +3094,7 @@ documented in additional help files.
     nimlsp................................|ale-nim-nimlsp|
     nimpretty.............................|ale-nim-nimpretty|
   nix.....................................|ale-nix-options|
+    alejandra.............................|ale-nix-alejandra|
     nixfmt................................|ale-nix-nixfmt|
     nixpkgs-fmt...........................|ale-nix-nixpkgs-fmt|
     statix................................|ale-nix-statix|