MDEV-35041 Simple comparison causes "Illegal mix of collations" even with default server settings

The task "MDEV-25829 Change default Unicode collation to uca1400_ai_ci"
previously changed collation derivation for string user variables
from DERIVATION_EXPLICIT to DERIVATION_COERCIBLE, to resolve illegal
collation mix conflicts between table columns and user variables
when they have different collations.

However, DERIVATION_COERCIBLE was a wrong choice because it caused
conflicts between string literals and user variables when they have
different collations.

Adding a new collation derivation level DERIVATION_USERVAR.
This makes the collation of a user variable:
- weaker than a table column (like it was intended by MDEV-25829)
- but stronger than a literal (like it was in pre-MDEV-25829)

Cleanup in sql_type.h:
  Removing the line "- BINARY(expr)" from the before-DERIVATION_CAST
  comment, as it was on a wrong place. It's also listed on the correct
  place before DERIVATION_IMPLICIT.

mysql-test/main/gis.result

@@ -4885,7 +4885,7 @@ UNCOMPRESSED_LENGTH(POINT(1,1))
 0
 SELECT COERCIBILITY(POINT(1,1));
 COERCIBILITY(POINT(1,1))
-5
+6
 SELECT ASCII(POINT(1,1));
 ASCII(POINT(1,1))
 0