add prettier fixer support for 'less' filetype

autoload/ale/fix/registry.vim

@@ -39,7 +39,7 @@ let s:default_registry = {
 \   },
 \   'prettier': {
 \       'function': 'ale#fixers#prettier#Fix',
-\       'suggested_filetypes': ['javascript', 'typescript', 'json', 'css', 'scss'],
+\       'suggested_filetypes': ['javascript', 'typescript', 'json', 'css', 'scss', 'less'],
 \       'description': 'Apply prettier to a file.',
 \   },
 \   'prettier_eslint': {

autoload/ale/fixers/prettier.vim

@@ -49,7 +49,7 @@ function! ale#fixers#prettier#Fix(buffer) abort
     if match(l:options, '--parser') == -1
         if l:filetype is# 'typescript'
             let l:parser = 'typescript'
-        elseif l:filetype =~# 'css\|scss'
+        elseif l:filetype =~# 'css\|scss\|less'
             let l:parser = 'postcss'
         elseif l:filetype is# 'json'
             let l:parser = 'json'

doc/ale.txt

@@ -95,6 +95,8 @@ CONTENTS                                                         *ale-contents*
     kotlin................................|ale-kotlin-options|
       kotlinc.............................|ale-kotlin-kotlinc|
       ktlint..............................|ale-kotlin-ktlint|
+    less..................................|ale-less-options|
+      prettier............................|ale-less-prettier|
     llvm..................................|ale-llvm-options|
       llc.................................|ale-llvm-llc|
     lua...................................|ale-lua-options|

test/fixers/test_prettier_fixer_callback.vader

@@ -114,3 +114,17 @@ Execute(Append '--parser postcss' for filetype=css):
   \     . ' --write',
   \ },
   \ ale#fixers#prettier#Fix(bufnr(''))
+
+Execute(Append '--parser postcss' for filetype=less):
+  set filetype=less
+  call ale#test#SetFilename('../prettier-test-files/testfile.less')
+
+  AssertEqual
+  \ {
+  \   'read_temporary_file': 1,
+  \   'command': ale#Escape(g:ale_javascript_prettier_executable)
+  \     . ' %t'
+  \     . ' --parser postcss'
+  \     . ' --write',
+  \ },
+  \ ale#fixers#prettier#Fix(bufnr(''))