refactor(nvim): use state dir for neovim undodir

Was using XDG_CACHE which is incorrect.

Yes, Neovim uses this by default, but it's nice to ensure this is always
set as I desire instead of an upstream change setting this differently.

dots/.config/nvim/lua/core/options.lua

@@ -58,7 +58,7 @@ M.setup = function()
     opt.signcolumn = "yes"
 
     -- Enable persistent undo
-    opt.undodir = vim.fn.stdpath("cache") .. "/undo"
+    opt.undodir = vim.fn.stdpath("state") .. "/undo"
     opt.undofile = true
 
     -- Better folding