feat(marksman): enable for markdown.mdx

`marksman` works for MDX files. We don't want to `set ft=markdown`,
since there exist tools that don't work for MDX, so we want to keep
those separate

lua/lspconfig/server_configurations/marksman.lua

@@ -6,7 +6,7 @@ local cmd = { bin_name, 'server' }
 return {
   default_config = {
     cmd = cmd,
-    filetypes = { 'markdown' },
+    filetypes = { 'markdown', 'markdown.mdx' },
     root_dir = function(fname)
       local root_files = { '.marksman.toml' }
       return util.root_pattern(unpack(root_files))(fname) or util.find_git_ancestor(fname)