feat: add option for view to follow cursor (#1727)

* feat: add option for custom entry view to follow cursor

Creates an option to allow the custom entries
view to follow the user's cursor as they type.

To enable, set
```lua
require("cmp").setup({
  view = {
    entries = {
      follow_cursor = true
    }
  }
})
```

Original source at 7569056388

Closes #1660

Co-authored-by: lvimuser <109605931+lvimuser@users.noreply.github.com>

* doc: add view.follow_cursor option to docs

---------

Co-authored-by: lvimuser <109605931+lvimuser@users.noreply.github.com>

lua/cmp/config/default.lua

@@ -97,6 +97,7 @@ return function()
       entries = {
         name = 'custom',
         selection_order = 'top_down',
+        follow_cursor = false,
       },
       docs = {
         auto_open = true,