Add is_insert_mode utility

lua/cmp/context.lua

@@ -65,12 +65,6 @@ context.new = function(prev_context, option)
   return self
 end
 
----Return the context is invalid or not.
----@return boolean
-context.invalid = function(self)
-  return string.sub(self.mode, 1, 1) ~= 'i'
-end
-
 ---Return context creation reason.
 ---@return cmp.ContextReason
 context.get_reason = function(self)