Fix #592 - desc index

[predictor2718]

Description

This PR fixes incorrect parsing of DESC inside index definitions in ALTER TABLE statements.

Example:

ALTER TABLE `t`
  ADD UNIQUE KEY `idx` (`a`, `b` DESC);

This change updates AlterOperation::parse() so that ASC and DESC are not interpreted as new statements within index definitions.

Fixes #592.

[predictor2718]

Thank you for the review!

Regarding the question about whether DeSC (or other mixed-case variants) would still trigger the check:

The parser normalizes keywords during lexing.
Lexer::parseKeyword() internally calls Context::isKeyword(), which performs a strtoupper() on the token value.
Because of this, any case variation like DESC, Desc, DeSC, etc. is always mapped to the uppercase keyword DESC.

So the condition is case-insensitive and mixed-case inputs won’t cause issues.

To be safe, I also added additional tests covering:

• ASC
• DESC
• lowercase desc
• two DESC columns

All tests pass with the current implementation.