chore(fmt): format using stylua

Also fixed something in .stylua.toml that may cause stylua to fail, by
the way

lua/outline/providers/markdown.lua

@@ -27,7 +27,7 @@ function M.supports_buffer(bufnr, config)
       end
     end
   end
-  return ft == "markdown"
+  return ft == 'markdown'
 end
 
 -- Parses markdown files and returns a table of SymbolInformation[] which is
@@ -47,8 +47,8 @@ function M.handle_markdown()
       goto nextline
     end
 
-    local next_value = lines[line+1]
-    local is_emtpy_line = #value:gsub("^%s*(.-)%s*$", "%1") == 0
+    local next_value = lines[line + 1]
+    local is_emtpy_line = #value:gsub('^%s*(.-)%s*$', '%1') == 0
 
     local header, title = string.match(value, '^(#+)%s+(.+)$')
     if not header and next_value and not is_emtpy_line then