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-supported-languages-and-tools.txt

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