OR REPLACE and IF NOT EXISTS in create trigger

<!-- 🚀 Thank you for contributing! --> <!-- Describe your changes clearly and use examples if possible. --> Added support `OR REPLACE` and `IF NOT EXISTS` for SQLCreateTrigger through builders. ```swift db.create(trigger: "foo", table: "planet", when: .before, event: .insert) .ifNotExists() ``` I made the function signature `public func ifNotExists(ifNotExists: Bool = true) -> Self ` but that's I wasn't sure whether to just set it to true or not, so I can change that if needed. <!-- When this PR is merged, the title and body will be --> <!-- used to generate a release automatically. -->