feat: tsql multiple add columns in alter statement

[matthias-Q]

Closes #135 and supersedes #144

I feel this is a bit hacky, but I just made the add keyword optional, so that it will not repeated in _alter_specifications

I also removed that rename part of the test, since this is not valid SQL syntax and made the keyword constrain optional. See discussion in #144

[dmfay]

it's a venial kludge at worst imo but a TODO comment above add_column might be a good idea in case we wind up revisiting