feat(markdown): add marksman lsp (#4565)

* feat(markdown): add marksman lsp
* docs: add marksman docs
* test(markdown): add marksman test

Co-authored-by: Peter Benjamin <peter.benjamin@peter.benjamin-FVFHP2WSQ05Q>

ale_linters/markdown/marksman.vim

@@ -0,0 +1,35 @@
+" Author: Peter Benjamin <petermbenjamin@gmail.com>
+" Description: Write Markdown with code assist and intelligence in the comfort of your favourite editor.
+
+call ale#Set('markdown_marksman_executable', 'marksman')
+
+function! ale_linters#markdown#marksman#GetCommand(buffer) abort
+    return '%e server'
+endfunction
+
+function! ale_linters#markdown#marksman#GetProjectRoot(buffer) abort
+    " Find nearest .marksman.toml
+    let l:marksman_toml = ale#path#FindNearestFile(a:buffer, '.marksman.toml')
+
+    if !empty(l:marksman_toml)
+        return fnamemodify(l:marksman_toml, ':h')
+    endif
+
+    " Find nearest .git/ directory
+    let l:project_root = finddir('.git/..', expand('#' . a:buffer . '...').';')
+
+    if !empty(l:project_root)
+        return l:project_root
+    endif
+
+    return ''
+endfunction
+
+call ale#linter#Define('markdown', {
+\   'name': 'marksman',
+\   'lsp': 'stdio',
+\   'executable': {b -> ale#Var(b, 'markdown_marksman_executable')},
+\   'command': function('ale_linters#markdown#marksman#GetCommand'),
+\   'project_root': function('ale_linters#markdown#marksman#GetProjectRoot'),
+\   'initialization_options': {},
+\})