Add erlfmt fixer to the registry and use it with stdin (#4868)

* Add erlfmt fixer to the registry

Without this, the fixer will not appear in the list of suggested tools
and cannot be used without additional configuration.

* Handle stdin in the erlfmt fixer command

Previously, the full path to the file being edited was used, which
resulted in the loss of unsaved changes.

* Add executable selection tests for erlfmt fixer

autoload/ale/fix/registry.vim

@@ -109,6 +109,11 @@ let s:default_registry = {
 \       'description': 'Indent with the Erlang mode for Emacs',
 \       'aliases': ['erlang-mode'],
 \   },
+\   'erlfmt': {
+\       'function': 'ale#fixers#erlfmt#Fix',
+\       'suggested_filetypes': ['erlang'],
+\       'description': 'Format Erlang code with erlfmt',
+\   },
 \   'fecs': {
 \       'function': 'ale#fixers#fecs#Fix',
 \       'suggested_filetypes': ['javascript', 'css', 'html'],

autoload/ale/fixers/erlfmt.vim

@@ -13,9 +13,7 @@ function! ale#fixers#erlfmt#Fix(buffer) abort
     let l:options = ale#Var(a:buffer, 'erlang_erlfmt_options')
     let l:executable = ale#fixers#erlfmt#GetExecutable(a:buffer)
 
-    let l:command = ale#Escape(l:executable) . (empty(l:options) ? '' : ' ' . l:options) . ' %s'
+    let l:command = ale#Escape(l:executable) . ale#Pad(l:options) . ' -'
 
-    return {
-    \   'command': l:command
-    \}
+    return {'command': l:command}
 endfunction