Add `$where()` method to `db.query`

[aleclarson]

This idea was suggested by Discord user @josefchi.

Essentially, it accepts the same value that db.query#findMany's where option does. But instead of executing a query, it returns a SQL instance, which you can use with db.select() calls, db.update() calls, etc.

Internally, it would call relationsFilterToSQL. Its main purpose is to provide type safety and auto-completion.

const where = db.query.user.$where({
  age: { gte: 18 },
})
// => [object SQL]

where.toSQL()
// => {
//   sql: `"user"."age" >= ?`,
//   params: [18],
// }