Added event_chars property to confirm_done event (#782)

* Added event_chars property to confirm_done event

Currently it seems there is no way for someone listening to the
`confirm_done` event to know exactly what character was used to commit
the completion confirmation.

This commit adds a `event_chars` property to the `confirm_done` event.
Its value is just the `chars` variable form the `core.on_keymap`
function.

* Renamed `commid_chars` to `commit_character`

lua/cmp/types/cmp.lua

@@ -38,6 +38,7 @@ cmp.ItemField.Menu = 'menu'
 
 ---@class cmp.ConfirmOption
 ---@field public behavior cmp.ConfirmBehavior
+---@field public commit_character? string
 
 ---@class cmp.SelectOption
 ---@field public behavior cmp.SelectBehavior