Statusline view in search mode or command line mode (#729)

* Search mode horizontal view

Enabled by setting `experimental.horizontal_search = true`

* use stylua

* move to a floating window instead of abusing status line

* pass all tests

* rework

1. rename entries view file and memeber to wildmenu_entries_view
2. move config to config.view.entries
3. support both in insert mode and cmdline
4. make separator configurable by config.view.separator

* rearange wildmenu config

Changed config to:
```lua
  view = {
    entries = {name = 'wildmenu', separator = '|' }
  },
```

* allow view.entries to be either a literal string or a table

lua/cmp/config/default.lua

@@ -123,5 +123,9 @@ return function()
     },
 
     sources = {},
+
+    view = {
+      entries = { name = 'custom' },
+    },
   }
 end