Add typehinting via py.typed

[piercefreeman]

I'm using pglast in some downstream projects and native typehinting support in my IDE would be pretty convenient. Happy to put up a PR for this functionality but wanted to check in on appetite before I do @lelit.

[lelit]

Hi, I"m taking a short break... Yes, I will appily merge your PR, especially if it comes with basic tests 😉 Bye, lele.

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