Add support for Packer (#4192)

* Add support for HashiCorp Packer

* Add test for packer fmt

* Add doc for HCL/Packer

* Add link to Packer doc

* Also suggest packer fix for packer ft

* Add more links to TOC

autoload/ale/fixers/packer.vim

@@ -0,0 +1,17 @@
+" Author: Zhuoyun Wei <wzyboy@wzyboy.org>
+" Description: Fixer for Packer HCL files
+
+call ale#Set('packer_fmt_executable', 'packer')
+call ale#Set('packer_fmt_options', '')
+
+function! ale#fixers#packer#Fix(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'packer_fmt_executable')
+    let l:options = ale#Var(a:buffer, 'packer_fmt_options')
+
+    return {
+    \   'command': ale#Escape(l:executable)
+    \       . ' fmt'
+    \       . (empty(l:options) ? '' : ' ' . l:options)
+    \       . ' -'
+    \}
+endfunction