Support date arithmetic with INTERVAL operators

[JanJakes]

Summary

This PR adds date arithmetic with INTERVAL, building on #510 by @samuelmbabhazi:

SELECT d + INTERVAL 1 DAY;
SELECT d - INTERVAL 5 HOUR;
SELECT d + INTERVAL 1 DAY - INTERVAL 2 HOUR;

These queries previously failed with a syntax error. They now use SQLite’s DATETIME() function, sharing the translation with DATE_ADD() and DATE_SUB().

Why the extra fixes

Our current parser groups some expressions differently from MySQL. Translating them directly can produce wrong results—or update or delete the wrong rows.

This PR adds safeguards to the implementation in #510:

• Reject expressions the parser cannot group correctly, including INTERVAL 1 DAY + d, 1 + d + INTERVAL 1 DAY, and d + INTERVAL 1 DAY + x. Errors suggest adding parentheses.
• Support QUARTER and SQL_TSI_* units, and handle negative and computed interval values.
• Reject unsupported units, including MICROSECOND and composite units such as HOUR_MINUTE, instead of silently returning NULL.

The supported forms cover the linked issues. The expression restrictions can be revisited after the planned switch to a parser based on MySQL’s own grammar.

Remaining limitations

The existing date translation still differs from MySQL in these cases:

• Numeric dates: Values such as 20240101 are not interpreted as MySQL dates.
• Interval values: MySQL converts the value to a whole number. Here, INTERVAL 1.5 DAY adds 36 hours instead of 2 days, and a string such as ' 1' returns NULL with + but works with -.
• Date-only inputs: These return datetime strings, so comparisons with DATE values can be wrong. For example, d < CURDATE() - INTERVAL 7 DAY also matches the boundary day.
• Time inputs: TIME values also return datetime strings, and times past midnight wrap around instead of going over 24 hours.
• Month ends: Month-end arithmetic can overflow into the following month.
• ADDDATE() and SUBDATE(): These functions remain unsupported.

Expected results in the new tests were checked against MySQL 8.4.

Fixes #454
Fixes #90

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