feat: support IS UNKNOWN as synonym to IS NULL

[systay]

Description

It seems like MySQL accepts IS UNKNOWN as a synonym of IS NULL. Here is a quick test that shows this on MySQL:

mysql> explain select 1 is unknown; show warnings;
<snip>
+-------+------+-----------------------------------------------------+
| Level | Code | Message                                             |
+-------+------+-----------------------------------------------------+
| Note  | 1003 | /* select#1 */ select (1 is null) AS `1 is unknown` |
+-------+------+-----------------------------------------------------+
1 row in set (0.00 sec)

As can be seen with this EXPLAIN/SHOW WARNINGS trick, MySQL rewrites IS UNKNOWN to IS NULL internally, so we'll do the same.

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.39%. Comparing base (4a1870a) to head (e9365a8).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15673      +/-   ##
==========================================
+ Coverage   68.13%   68.39%   +0.26%     
==========================================
  Files        1556     1556              
  Lines      195028   195028              
==========================================
+ Hits       132888   133398     +510     
+ Misses      62140    61630     -510     

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