feat: add IF EXISTS support for alter table

[aryanghugare]

Implementation: ALTER TABLE IF EXISTS support

Added ifExists() method to AlterTableBuilder, following the same pattern as DropTableBuilder.ifExists() and CreateTableBuilder.ifNotExists().

Usage:

db.schema.alterTable('person').ifExists().addColumn('age', 'integer')
// PostgreSQL: alter table if exists "person" add column "age" integer

Files changed:

1. src/operation-node/alter-table-node.ts — Added ifExists?: boolean to AlterTableNode interface and AlterTableNodeTableProps
2. src/schema/alter-table-builder.ts — Added ifExists() chainable method on AlterTableBuilder
3. src/query-compiler/default-query-compiler.ts — Emit "if exists" in visitAlterTable() when flag is set
4. src/operation-node/operation-node-transformer.ts — Pass through ifExists in transformAlterTable()
5. test/node/src/schema.test.ts — Added test covering Postgres, MySQL, and SQLite output

Happy to open a PR if this approach looks good!
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commit: edbf5f2

[aryanghugare]

Bumping this. This is pretty critical for idempotent alterations.

Thanks for the bump! Happy to make any further changes if needed.