fix: Adds erlang:timestamp() to typespec

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This is supported in jsx_parser which calls calendar:now_to_datetime/1 for these timestamps.

Opened a PR here as well but it seems like this project is not alive any more.

The reason:

The call jsx:encode(Timestamp::{_,_,_}) will never return since the success typing is
(atom() | binary() | [any()] | number() | {{non_neg_integer(),1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12,1..255},{byte(),byte(),byte()}} | map()) -> binary()
and the contract is (Source::json_term()) -> json_text()

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This commit has been added on top of 2.9.0 here: https://github.com/kivra/jsx/commits/2.9.0-kivra