Fix markdownlint fixer and add markdownlint-cli2 (#5165)

* 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

* fix: wrong command for markdownlint

* feat: explicit support for markdownlint-cli2

- Add markdownlint-cli2 linter and fixer, separate from markdownilnt-cli
- Rename markdownlint to markdownlint-cli
- Keep markdownlint as alias for markdownlint-cli

---------

Co-authored-by: w0rp <devw0rp@gmail.com>
Co-authored-by: Horacio Sanson <horacio.sanson@dena.jp>

doc/ale.txt

@@ -3780,7 +3780,8 @@ documented in additional help files.
     cspell................................|ale-markdown-cspell|
     dprint................................|ale-markdown-dprint|
     harper................................|ale-markdown-harper|
-    markdownlint..........................|ale-markdown-markdownlint|
+    markdownlint-cli......................|ale-markdown-markdownlint-cli|
+    markdownlint-cli2.....................|ale-markdown-markdownlint-cli2|
     marksman..............................|ale-markdown-marksman|
     mdformat..............................|ale-markdown-mdformat|
     mdl...................................|ale-markdown-mdl|