chore(render-markdown): use API to toggle

## Details

The `state` module in `render-markdown` is not expected to be accessed
outside of the package and may be refactored in the future.

Prevent any breaking changes in LazyVim by using the public API which
was updated here:
accaa600ba

lua/lazyvim/plugins/extras/lang/markdown.lua

@@ -113,17 +113,8 @@ return {
       require("render-markdown").setup(opts)
       Snacks.toggle({
         name = "Render Markdown",
-        get = function()
-          return require("render-markdown.state").enabled
-        end,
-        set = function(enabled)
-          local m = require("render-markdown")
-          if enabled then
-            m.enable()
-          else
-            m.disable()
-          end
-        end,
+        get = require("render-markdown").get,
+        set = require("render-markdown").set,
       }):map("<leader>um")
     end,
   },