MDEV-11347 Move add_create_index_prepare(), add_key_to_list(), set_trigger_new_row(), set_local_variable(), set_system_variable(), create_item_for_sp_var() as methods to LEX

Moving another banch of functions implemented in sql_yacc.yy as methods to LEX,
to be able to reuse them between sql_yacc.yy and sql_yacc_ora.yy easier.
The list of functions:
- add_create_index_prepare()
- add_key_to_list()
- set_trigger_new_row()
- set_local_variable()
- set_system_variable()
- create_item_for_sp_var()