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:/…)"

test/functional/lua/highlight_spec.lua

@@ -43,9 +43,9 @@ describe('vim.highlight.on_yank', function()
       vim.api.nvim_buf_set_mark(0,"]",1,1,{})
       vim.highlight.on_yank({timeout = math.huge, on_macro = true, event = {operator = "y"}})
     ]])
-    neq({}, api.nvim_win_get_ns(0))
+    neq({}, api.nvim__win_get_ns(0))
     command('wincmd w')
-    eq({}, api.nvim_win_get_ns(0))
+    eq({}, api.nvim__win_get_ns(0))
   end)
 
   it('removes old highlight if new one is created before old one times out', function()
@@ -55,7 +55,7 @@ describe('vim.highlight.on_yank', function()
       vim.api.nvim_buf_set_mark(0,"]",1,1,{})
       vim.highlight.on_yank({timeout = math.huge, on_macro = true, event = {operator = "y"}})
     ]])
-    neq({}, api.nvim_win_get_ns(0))
+    neq({}, api.nvim__win_get_ns(0))
     command('wincmd w')
     exec_lua([[
       vim.api.nvim_buf_set_mark(0,"[",1,1,{})
@@ -63,6 +63,6 @@ describe('vim.highlight.on_yank', function()
       vim.highlight.on_yank({timeout = math.huge, on_macro = true, event = {operator = "y"}})
     ]])
     command('wincmd w')
-    eq({}, api.nvim_win_get_ns(0))
+    eq({}, api.nvim__win_get_ns(0))
   end)
 end)