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

---------

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

doc/ale.txt

@@ -3779,6 +3779,7 @@ documented in additional help files.
     harper................................|ale-markdown-harper|
     markdownlint..........................|ale-markdown-markdownlint|
     marksman..............................|ale-markdown-marksman|
+    mdformat..............................|ale-markdown-mdformat|
     mdl...................................|ale-markdown-mdl|
     pandoc................................|ale-markdown-pandoc|
     prettier..............................|ale-markdown-prettier|