Add alejandra for nix (#4435)

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

autoload/ale/fix/registry.vim

@@ -7,6 +7,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['python'],
 \       'description': 'Add blank lines before control statements.',
 \   },
+\   'alejandra': {
+\       'function': 'ale#fixers#alejandra#Fix',
+\       'suggested_filetypes': ['nix'],
+\       'description': 'The Uncompromising Nix Code Formatter',
+\   },
 \   'align_help_tags': {
 \       'function': 'ale#fixers#help#AlignTags',
 \       'suggested_filetypes': ['help'],

autoload/ale/fixers/alejandra.vim

@@ -0,0 +1,13 @@
+call ale#Set('nix_alejandra_executable', 'alejandra')
+call ale#Set('nix_alejandra_options', '')
+
+function! ale#fixers#alejandra#Fix(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'nix_alejandra_executable')
+    let l:options = ale#Var(a:buffer, 'nix_alejandra_options')
+
+    return {
+    \   'command': ale#Escape(l:executable)
+    \       . (empty(l:options) ? '' : ' ' . l:options)
+    \       . ' -- -'
+    \}
+endfunction