Support NVARCHAR

[lem9]

See https://dev.mysql.com/doc/refman/5.7/en/string-type-syntax.html and https://stackoverflow.com/questions/44212793/how-to-make-nvarchar-column-in-phpmyadmin

[ibennetch]

Thanks Marc for the report.

These are the syntax variations we need to accept (copied from the manual page):

CHAR(10) CHARACTER SET utf8
NATIONAL CHARACTER(10)
NCHAR(10)

and

VARCHAR(10) CHARACTER SET utf8
NATIONAL VARCHAR(10)
NVARCHAR(10)
NCHAR VARCHAR(10)
NATIONAL CHARACTER VARYING(10)
NATIONAL CHAR VARYING(10)

Also: "You can use N'literal' (or n'literal') to create a string in the national character set."

SELECT N'some text';
SELECT n'some text';
SELECT _utf8'some text';