DBForge create_database if not exists

Like `create_table`, the `create_database` method should have `$exists = false`
Such that:
`create_database("name", true)` will render a "CREATE DATABASE `name` IF NOT EXISTS"
and in case it returns an error, will NOT call "show_error" (which exists the page)

This is changing just a few lines of code, with not DB engine compatibility issue to my knowledge

Well, there are databases that don't support this ... we have to go through the docs of all platforms and see which ones do and which ones don't.
Other than that, yes - I don't see a problem with this either.

Thanks. Because there were no objections, I took the liberty to create a pull request