refactor(api): nvim_win_xx_ns are EXPERIMENTAL

Problem:
The nvim_win_xx_ns function family introduced in ba0370b1d7
needs more bake-time. Currently it's narrowly defined for windows, but
other scopes ("buffer") and features are likely in the future.

Solution:
- Rename the API with double-underscore to mark it as EXPERIMENTAL.

TODO/FUTURE:
- Rename and change the signature to support more than just "window"
  scope, and for other flexibility.
- Open question: we could choose either:
  - "store scopes on namespaces", or
  - "store namespaces on scopes (w:/b:/…)"

runtime/lua/vim/highlight.lua

@@ -129,7 +129,7 @@ function M.on_yank(opts)
     yank_cancel()
   end
 
-  vim.api.nvim_win_add_ns(winid, yank_ns)
+  vim.api.nvim__win_add_ns(winid, yank_ns)
   M.range(bufnr, yank_ns, higroup, "'[", "']", {
     regtype = event.regtype,
     inclusive = event.inclusive,
@@ -141,7 +141,7 @@ function M.on_yank(opts)
     yank_timer = nil
     yank_cancel = nil
     pcall(vim.api.nvim_buf_clear_namespace, bufnr, yank_ns, 0, -1)
-    pcall(vim.api.nvim_win_del_ns, winid, yank_ns)
+    pcall(vim.api.nvim__win_del_ns, winid, yank_ns)
   end
 
   yank_timer = vim.defer_fn(yank_cancel, timeout)