[Feature] Allow window function modification

[Azarattum]

Originally discussed in https://github.com/kysely-org/kysely/issues/397#issuecomment-1553177254. This would allow support for the frame spec of SQLite. As suggested by @igalklebanov in https://github.com/kysely-org/kysely/issues/397#issuecomment-1553239805 a new syntax .modifyEnd and .modifyFront could be added to OverBuilder to match SelectQueryBuilder syntax. For example an unbounded window would look something like this:

fn.over(qb => 
  qb
    .partitionBy("group")
    .orderBy("order")
    .modifyEnd(sql`ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING`)
)

[igalklebanov]

Hey 👋

Thanks!

This is also relevant to MySQL & PostgreSQL, and named "frame clause" in both.