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/types/cmp.lua

@@ -184,6 +184,7 @@ cmp.ItemField = {
 ---@class cmp.CustomEntriesViewConfig
 ---@field name 'custom'
 ---@field selection_order 'top_down'|'near_cursor'
+---@field follow_cursor boolean
 
 ---@class cmp.NativeEntriesViewConfig
 ---@field name 'native'