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>

test/fixers/test_mdformat_fixer_callback.vader

@@ -0,0 +1,46 @@
+Before:
+  call ale#assert#SetUpFixerTest('markdown', 'mdformat')
+
+After:
+  call ale#assert#TearDownFixerTest()
+
+Execute(The mdformat callback should return the default command):
+  AssertFixer
+  \ {
+  \   'command': ale#Escape('mdformat') . ' -',
+  \ }
+
+Execute(The mdformat executable and options should be configurable):
+  let g:ale_markdown_mdformat_executable = 'foobar'
+  let g:ale_markdown_mdformat_options = '--wrap 80'
+
+  AssertFixer
+  \ {
+  \   'command': ale#Escape('foobar') . ' --wrap 80 -',
+  \ }
+
+Execute(Setting executable to pipenv should append run mdformat):
+  let g:ale_markdown_mdformat_executable = 'path/to/pipenv'
+
+  AssertFixer
+  \ {
+  \   'command': ale#Escape('path/to/pipenv') . ' run mdformat -',
+  \ }
+
+Execute(Poetry should be detected when auto_poetry is set):
+  let g:ale_markdown_mdformat_auto_poetry = 1
+  call ale#test#SetFilename('../test-files/python/poetry/whatever.py')
+
+  AssertFixer
+  \ {
+  \   'command': ale#Escape('poetry') . ' run mdformat -',
+  \ }
+
+Execute(uv should be detected when auto_uv is set):
+  let g:ale_markdown_mdformat_auto_uv = 1
+  call ale#test#SetFilename('../test-files/python/uv/whatever.py')
+
+  AssertFixer
+  \ {
+  \   'command': ale#Escape('uv') . ' run mdformat -',
+  \ }