refactor: Extract AlterExpressionAddAlterModify and AlterExpressionRenameOp

AlterExpression() shrinks from 754 to 211 lines, now a clean dispatcher.

AlterExpressionAddAlterModify (185 lines): all ADD/ALTER/MODIFY logic
including PRIMARY KEY, INDEX, SPATIAL/FULLTEXT, column definitions,
UNIQUE, FOREIGN KEY, CHECK ENFORCED, and CONSTRAINT.

AlterExpressionRenameOp (47 lines): unifies three scattered RENAME
variants (column, table, index/key/constraint) into one production.

Also: CreateTable.toString() refactored to StringBuilder + helpers,
CheckConstraint.toString() fixed for unnamed constraints (was emitting
"CONSTRAINT null").

Signed-off-by: Andreas Reichel <andreas@manticore-projects.com>
Signed-off-by: manticore-projects <andreas@manticore-projects.com>

Author: manticore-projects