Support MySQL index column length prefix syntax in CREATE TABLE UNIQUE constraint

[Copilot]

AlaSQL fails to parse MySQL's index length prefix syntax (col(N)) inside UNIQUE constraints in CREATE TABLE statements, throwing a parse error at the LPAR token.

Changes

• Grammar (src/alasqlparser.jison): Introduce an IndexColumnsList rule that accepts both plain column names (Literal) and column names with an optional length prefix (Literal LPAR Number RPAR). Update UniqueKey to use IndexColumnsList instead of OrderExpressionsList.
• Parser (src/alasqlparser.js): Regenerated from updated .jison grammar via yarn jison.
• Tests (test/test406.js): Added regression tests covering single column with length prefix, multiple columns with length prefixes, and mixed columns (with and without prefix).

Example

-- Previously threw: Parse error ... got 'LPAR'
CREATE TABLE t7(a TEXT, UNIQUE (a(1)));

-- Multiple columns with length prefixes
CREATE TABLE t8(a TEXT, b TEXT, UNIQUE (a(1), b(2)));

-- Mixed: length-prefixed and plain columns
CREATE TABLE t9(a TEXT, b INT, UNIQUE (a(5), b));

• Fixes Parse error CREATE TABLE UNIQUE (a(1)) #406