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.

doc/ale-nickel.txt

@@ -0,0 +1,25 @@
+===============================================================================
+ALE Nickel Integration                                      *ale-nickel-options*
+
+
+===============================================================================
+nickel_format                                         *ale-nickel-nickel-format*
+
+g:ale_nickel_nickel_format_executable    *g:ale_nickel_nickel_format_executable*
+                                         *b:ale_nickel_nickel_format_executable*
+  Type: |String|
+  Default: `'nickel'`
+
+  This option can be changed to change the path for `nickel`.
+
+
+g:ale_nickel_nickel_format_options          *g:ale_nickel_nickel_format_options*
+                                            *b:ale_nickel_nickel_format_options*
+  Type: |String|
+  Default: `''`
+
+  This option can be changed to pass extra options to `'nickel format'`
+
+
+===============================================================================
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

doc/ale-supported-languages-and-tools.txt

@@ -382,6 +382,8 @@ Notes:
   * `mmc`!!
 * NASM
   * `nasm`!!
+* Nickel
+  * `nickel_format`
 * Nim
   * `nim check`!!
   * `nimlsp`

doc/ale.txt

@@ -3188,6 +3188,8 @@ documented in additional help files.
     mmc...................................|ale-mercury-mmc|
   nasm....................................|ale-nasm-options|
     nasm..................................|ale-nasm-nasm|
+  nickel..................................|ale-nickel-options|
+    nickel_format.........................|ale-nickel-nickel-format|
   nim.....................................|ale-nim-options|
     nimcheck..............................|ale-nim-nimcheck|
     nimlsp................................|ale-nim-nimlsp|