@DatabaseFunction: Custom SQLite Functions

[stephencelis]

This PR introduces the ability to create custom functions for SQLite. You can apply a new @DatabaseFunction macro to a Swift function and a type-safe query helper is generated alongside it.

You could control a date dependency, for example:

@DatabaseFunction
func customDate() -> Date {
  @Dependency(\.date.now) var now
  return now
}

// In a query:
Reminder.where { $0.dueDate <= $customDate() }
// SELECT … FROM "reminders" WHERE "dueDate" <= "customDate"()

// Theoretical syntax for adapters like SharingGRDB:
db.install($customDate)

This functionality is implemented in a new StructuredQueriesSQLite module, which we can use to quarantine SQLite-specific functionality. We should also move the FTS stuff there soon.

[stephencelis]

I think we theoretically could support variadics (and maybe even parameter packs) with a bit more work, but maybe we wait for some good use cases.