add: support for nickel format as Nickel fixer (#4665)

Nickel(https://nickel-lang.org/) is a configuration language, like
Jsonnet, Cue, Dhall.

`nickel`(https://github.com/tweag/nickel) is the main command to run,
export and also format Nickel code.

this commit adds `nickel format` as a Nickel fixer, together with some
tests and documentation.

autoload/ale/fix/registry.vim

@@ -635,7 +635,12 @@ let s:default_registry = {
 \       'function': 'ale#fixers#erbformatter#Fix',
 \       'suggested_filetypes': ['eruby'],
 \       'description': 'Apply erb-formatter -w to eruby/erb files.',
-\   }
+\   },
+\   'nickel_format': {
+\       'function': 'ale#fixers#nickel_format#Fix',
+\       'suggested_filetypes': ['nickel'],
+\       'description': 'Fix nickel files with nickel format',
+\   },
 \}
 
 " Reset the function registry to the default entries.