Add warnings for data/time/datetime delimiters as well as superfluous characters/spaces.

## Enhancement

Generate warnings if:
- [ ] DATE delimiters are not '-'
- [ ] TIME delimiters are not ':'
- [ ] delimiter between DATE and TIME is not a single space character
- [ ] extra whitespaces prefixes or suffixes

Maybe also check MySQL acceptable formats like '10101.5' (MySQL interpret it as '2010-10-01 05:00:00') or '4.12.3' (MySQL interpret it as '2004-12-03 00:00:00', while '4-12-3' translates to '0004-12-03 00:00:00') or at least make sure there are tests and it is documented (feel free to move this to a separate issue)

From #51742 to have it as a separate issue:
```mysql
mysql-9.0.1> SELECT DATE'2012@12@31';
+------------------+
| DATE'2012@12@31' |
+------------------+
| 2012-12-31 |
+------------------+
1 row in set, 1 warning (0.00 sec)

mysql-9.0.1> SHOW WARNINGS;
+---------+------+-------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------------------------------------------------------------------+
| Warning | 4095 | Delimiter '@' in position 4 in datetime value '2012@12@31' at row 1 is deprecated. Prefer the standard '-'. |
+---------+------+-------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
tidb> SELECT DATE'2012@12@31';
+------------------+
| DATE'2012@12@31' |
+------------------+
| 2012-12-31 |
+------------------+
1 row in set (0.00 sec)
```

No files, tests, or entry points are named. Start by locating TiDB's DATE, TIME, and DATETIME literal parsing and its existing warning tests; use the listed delimiter and whitespace cases as acceptance criteria, while treating the optional MySQL-format cases as separate scope or documentation.

Written by the indexing model from the issue text.