Add support for syntax_tree fixer (#4268)

This is the library that now powers prettier/plugin-ruby but is also
stands on its own: https://github.com/ruby-syntax-tree/syntax_tree

doc/ale-ruby.txt

@@ -219,5 +219,25 @@ g:ale_ruby_standardrb_options                   *g:ale_ruby_standardrb_options*
   This variable can be changed to modify flags given to standardrb.
 
 
+===============================================================================
+syntax_tree                                              *ale-ruby-syntax_tree*
+
+g:ale_ruby_syntax_tree_executable           *g:ale_ruby_syntax_tree_executable*
+                                            *b:ale_ruby_syntax_tree_executable*
+  Type: String
+  Default: `'stree'`
+
+  Override the invoked SyntaxTree binary. Set this to `'bundle'` to invoke
+  `'bundle` `exec` stree'.
+
+
+g:ale_ruby_syntax_tree_options                 *g:ale_ruby_syntax_tree_options*
+                                               *b:ale_ruby_syntax_tree_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be changed to modify flags given to SyntaxTree.
+
+
 ===============================================================================
   vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

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

@@ -529,6 +529,7 @@ Notes:
   * `solargraph`
   * `sorbet`
   * `standardrb`
+  * `syntax_tree`
 * Rust
   * `cargo`!!
   * `cspell`

doc/ale.txt

@@ -3167,6 +3167,7 @@ documented in additional help files.
     solargraph............................|ale-ruby-solargraph|
     sorbet................................|ale-ruby-sorbet|
     standardrb............................|ale-ruby-standardrb|
+    syntax_tree...........................|ale-ruby-syntax_tree|
   rust....................................|ale-rust-options|
     analyzer..............................|ale-rust-analyzer|
     cargo.................................|ale-rust-cargo|