parser: fix SQL_TSI_YEAR column type syntax

[hey-kong]

What problem does this PR solve?

Fix compatibility problem about SQL_TSI_YEAR column type syntax

Issue: pingcap/parser#537

MySQL Syntax:

type:
...
        | YEAR_SYM opt_field_length field_options
          {
            if ($2)
            {
              errno= 0;
              ulong length= strtoul($2, NULL, 10);
              if (errno != 0 || length != 4)
              {
                /* Only support length is 4 */
                my_error(ER_INVALID_YEAR_COLUMN_LENGTH, MYF(0), "YEAR");
                MYSQL_YYABORT;
              }
            }
            // We can ignore field length and UNSIGNED/ZEROFILL attributes here.
            $$= NEW_PTN PT_year_type;
          }

Bad SQL Case:

CREATE TABLE IF NOT EXISTS table_ident (a SQL_TSI_YEAR(4), b SQL_TSI_YEAR)
CREATE TABLE IF NOT EXISTS table_ident (ident1 BOOL COMMENT "text_string" unique, ident2 SQL_TSI_YEAR(4) ZEROFILL)

Check List

Tests

• Unit test

[codecov]

Codecov Report

Merging #540 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #540   +/-   ##
=======================================
  Coverage   71.77%   71.77%           
=======================================
  Files          32       32           
  Lines        7802     7802           
=======================================
  Hits         5600     5600           
  Misses       1674     1674           
  Partials      528      528

Impacted Files	Coverage Δ
parser.go	70.58% <ø> (ø)	⬆️

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[tangenta]

LGTM

[kennytm]

LGTM