feat: Add union operator

[max-sixty]

This is still a WIP -- I'm getting a error in semantic which I need to spend some more time on.

Any thoughts on the language design? Currently this has a required positional param to specify the type of the union -- should we do that vs. an optional named param?

[aljazerzen]

I like the language design: in most cases you'd write union all, which is quite self-explanatory. We could use a default value here, but we'd have to convert to a named argument:

from table1
union kind:distinct table2

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Regarding the comment about resolving: to be honest I don't remember how to enable table resolving at main branch because I redid that at semantic branch 2 times :D Can this PR wait a week for semnatic to be merged?

[max-sixty]

Can this PR wait a week for semantic to be merged?

For sure!

[aljazerzen]

I've tried to convert this to a "good-first-issue", but it included work on many parts of the codebase, so I just did it myself.

It needs docs, though.

This is how it works:

from employees
concat managers
union other_employees

[max-sixty]

Awesome!! I am happy to do the docs