Correct typing for startedAs and virtualAs database column definitions

[ollieread]

This PR changes the typing of ColumnDefinition::storedAs() and ColumnDefinition::virtualAs() to include \Illuminate\Database\Query\Expression and not just string.

This has been done because ColumnDefinition::generatedAs() accepts the type string|\Illuminate\Database\Query\Expression, and after digging through the code that creates the columns, all three types are capable of handling expressions.

This is QOL PR that just makes Laravels typing a bit nicer to work with.

[crynobone]

Any specific reason we type-hinting to implementation Illuminate\Database\Query\Expression instead of the interface Illuminate\Contracts\Database\Query\Expression?

[ollieread]

Any specific reason we type-hinting to implementation Illuminate\Database\Query\Expression instead of the interface Illuminate\Contracts\Database\Query\Expression?

That's the value used by generatedAs(), which I suspect is a layover from when we didn't have the contract. I'll update.