Added setNullable support to SQLite

[aidenfoxx]

As discussed in #4681 I've added support for setNullable and dropNullable to the SQLite dialect.

Hopefully I didn't miss anything!

[aidenfoxx]

Just a question for @nickrum when he gets a chance:

In #ed0e8a I notice you updated the alterColumn DDL to use this.alter rather than this.generateAlterCommands. Judging by the commit message, there is a good reason for the change. However, @kibertoad encouraged me to use this.generateAlterCommands since it returns the distinct statements of the function.

Anyway, my point is, which is correct way of doing things since there is clearly issues with both ways?

[kibertoad]

@aidenfoxx Judging by skipped test, it looks more like a hack than a proper solution. I still believe that generateAlterCommands is a better choice.

[nickrum]

@aidenfoxx The problem with generateAlterCommands was that the generated commands weren't executed in a transaction, so if there was an error while executing one of the alter statements, the database might be left in some in-between-state without a way to revert back. This was IMO worse than not being able to generate the commands.

Now that #4189 has been merged, using generateAlterCommands should be as safe as using alter, so I reverted the hotfix in #4896.