feat(markdown): add rumdl server and fixer (#5115)

* feat: put in the main linter files

* feat: add to registry

* docs: add rumdl to docs

* tests: vader tests

* edit: actually split the options into two

* style: make rumdl fixer test mimic markdownlint

* fix: stupidity overwhelming

* fix: actually let's look for pyproject too

copied from tombi

* i'm a buffoon fix wrong indentation

* alignment ci is made for people like me

* missed toc entry

test/fixers/test_rumdl_fixer_callback.vader

@@ -0,0 +1,24 @@
+Before:
+  call ale#assert#SetUpFixerTest('markdown', 'rumdl')
+
+After:
+  call ale#assert#TearDownFixerTest()
+
+Execute:
+  AssertFixer {
+  \ 'command': ale#Escape('rumdl') . ' fmt - --silent',
+  \}
+
+Execute:
+  let g:ale_markdown_rumdl_executable = 'rumdl_custom'
+
+  AssertFixer {
+  \ 'command': ale#Escape('rumdl_custom') . ' fmt - --silent',
+  \}
+
+Execute:
+  let g:ale_markdown_rumdl_fmt_options = ''
+
+  AssertFixer {
+  \ 'command': ale#Escape('rumdl') . ' fmt -',
+  \}

test/linter/test_markdown_rumdl.vader

@@ -0,0 +1,18 @@
+Before:
+  call ale#assert#SetUpLinterTest('markdown', 'rumdl')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default command should be correct):
+  AssertLinter 'rumdl', ale#Escape('rumdl') . ' server --stdio'
+
+Execute(The executable should be configurable):
+  let b:ale_markdown_rumdl_executable = 'rumdl_custom'
+
+  AssertLinter 'rumdl_custom', ale#Escape('rumdl_custom') . ' server --stdio'
+
+Execute(The server options should be configurable):
+  let b:ale_markdown_rumdl_options = '--some-flag'
+
+  AssertLinter 'rumdl', ale#Escape('rumdl') . ' server --stdio --some-flag'