[Feature] Allow creating indexes using ALTER TABLE ... ADD INDEX

[Gaspero]

Currently the only way of creating indexes using Kysely is using createIndex method e.g.:

await db.schema
    .createIndex('user_email_index')
    .on('users_test')
    .column('email')
    .execute()

But MySQL and some other non-standard dialects (YDB for example) also support creating indexes using
ALTER TABLE ... ADD INDEX expressions.

Please consider adding support for creating indexes this way.