feat(ui): allow to set the highlight namespace per window

- reimplement 'winhl' in terms of highlight namespaces
- check for EOF in screen tests (to indicate a likely crash)

test/functional/api/highlight_spec.lua

@@ -243,7 +243,7 @@ describe("API: set highlight", function()
 
   local function get_ns()
     local ns = meths.create_namespace('Test_set_hl')
-    meths._set_hl_ns(ns)
+    meths.set_hl_ns(ns)
     return ns
   end