'Wanna a drink' is wrong, forget about it. It doesn't mean 'want to' here. If you hear someone saying 'wanna drink?' what they are saying is 'want a drink?' quickly, which is an informal way of asking the question 'do you want a drink?' 'Wanna is how it sounds when native speakers speak quickly. My advice would to be recognise what is meant by it but not try and intentionally use it yourself for what it's worth. Native speakers do not intentionally say 'wanna' when they speak, that's just how it sounds when spoken rapidly. Consequently, if a learner speaker does try and say 'wanna' on purpose it sounds weird and off. And as you demonstrated with your question, there are pitfalls when people don't understand its usage properly. Yes, 'drink' can refer to any beverage. The context would make it clearer. If you're in a pub and someone asks you if you want a drink then obviously it's a lot more likely that you're going to want an alcoholic drink and not a soft drink.

"Wanna" is a contraction in English for "want to" or "want a." In day-to-day English you would say "wanna drink" NOT wanna a drink. Remember this would normally be informal talk. Yes a drink can refer to non-alcoholic or alcoholic drinks. You might clarify by saying, for example, "wanna beer" or "wanna drink of pop." The other person might just ask you what you what it is that you are offering to give him or her as a drink.

No you can't say "I wanna a drink" since "wanna" is "want to". So you can only say "I want a drink." but when someone's speaking really fast (especially a native speaker) it may sound like "I wanna drink." And yes the word "drink" doesn't imply whether you mean a alcoholic drink or a non-alcoholic one. The person you're saying this sentence to, will probably ask you what exactly you want him/her to bring you.

wanna (informal, non-standard) (=the written form of the word some people use to mean ‘want to’ or ‘want a’, which is not considered to be correct) I wanna go. Wanna drink? (= Do you want…) You should not write this form unless you are copying somebody’s speech. I don't wanna go into detail yet, I want you guys to just see it and then give me your feedback please!

I suggest that you try not to use "wanna" in written English unless you are writing an informal dialog. In the United States it is very common to pronounce "want to" as "wanna" but it looks horrible when it is written. Unless you are certain of your audience, it is safer to use "want to".