refactor: set concealcursor=nvic by default

Originally I set the default value to `n` because I wanted users to be
aware of the magic ID number at the start of the line if they were
editing a buffer. With the recent improvements to the cursor placement,
I think that it's no longer necessary. Most normal edits should just
work as expected now, and it's a nicer experience if the visuals don't
jump around when you enter visual/insert mode.

lua/oil/config.lua

@@ -24,7 +24,7 @@ local default_config = {
     spell = false,
     list = false,
     conceallevel = 3,
-    concealcursor = "n",
+    concealcursor = "nvic",
   },
   -- Restore window options to previous values when leaving an oil buffer
   restore_win_options = true,