add: support for rubyfmt as ruby fixer (#4678)

[rubyfmt](https://github.com/fables-tales/rubyfmt) is a formatter for
 `ruby` code.

This commit adds support for `rubyfmt` as a `ruby` fixer (#2991),
together with some tests and documentation.

doc/ale-ruby.txt

@@ -258,6 +258,24 @@ g:ale_ruby_syntax_tree_options                 *g:ale_ruby_syntax_tree_options*
 
   This variable can be changed to modify flags given to SyntaxTree.
 
+===============================================================================
+rubyfmt                                                       *ale-ruby-rubyfmt*
+
+g:ale_ruby_rubyfmt_executable                    *g:ale_ruby_rubyfmt_executable*
+                                                 *b:ale_ruby_rubyfmt_executable*
+  Type: |String|
+  Default: `'rubyfmt'`
+
+  This option can be changed to change the path for `rubyfmt`.
+
+
+g:ale_ruby_rubyfmt_options                          *g:ale_ruby_rubyfmt_options*
+                                                    *b:ale_ruby_rubyfmt_options*
+  Type: |String|
+  Default: `''`
+
+  This option can be changed to pass extra options to `'rubyfmt'`.
+
 
 ===============================================================================
   vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

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

@@ -560,6 +560,7 @@ Notes:
   * `reek`
   * `rubocop`
   * `ruby`
+  * `rubyfmt`
   * `rufo`
   * `solargraph`
   * `sorbet`

doc/ale.txt

@@ -3352,6 +3352,7 @@ documented in additional help files.
     sorbet................................|ale-ruby-sorbet|
     standardrb............................|ale-ruby-standardrb|
     syntax_tree...........................|ale-ruby-syntax_tree|
+    rubyfmt...............................|ale-ruby-rubyfmt|
   rust....................................|ale-rust-options|
     analyzer..............................|ale-rust-analyzer|
     cargo.................................|ale-rust-cargo|