FULL OUTER JOIN: Make the parser support FULL OUTER JOIN syntax

Taking a part of MDEV-36813:

Fix the parser. (an attempt to just add FULL OUTER JOIN alongside with LEFT and INNER and that causes some mysterious reduce/reduce conflicts: full-outer-join-parser-FAIL.diff ).

The produced data structure for

  (t1 FULL OUTER JOIN t2 ON ... )

TABLE_LIST(t1) will have a mark noting it's left table of full outer join
TABLE_LIST(t2) will have a mark noting it's right table of full outer join and on_expr (like right tables of LEFT JOINs do).

In the first milestone, you can also introduce/set LEX::has_full_outer_join and produce ER_NOT_SUPPORTED (instead of Parse Error) when that variable is set.