5151 add mdformat support (#5152)

* fix(tests): fix ale_c_build_dir_names being unset in tests (#5109)

- Use ale#Set() to set the ale_c_build_dir_names variable.
- Ensure SetUpLinterTest() is called before any Save commands in tests.
- Add c.vim to runtime before non-linter tests are executed.
- Remove workarounds in c.vim.

* feat: Load Prettier from cjs also

* Add support for mdformat

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Co-authored-by: w0rp <devw0rp@gmail.com>

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

@@ -436,6 +436,7 @@ Notes:
   * `languagetool`!!
   * `markdownlint`!!
   * `marksman`
+  * `mdformat`
   * `mdl`
   * `pandoc`
   * `prettier`